International Association of Geodesy

Geodynamics Section 1999-2003
Clark R. Wilson, President

1.Overview 

With the reorganization of the IAG to take effect at the meeting in Sapporo, this report is a final one for the Geodynamics Section V of the IAG. In the future, Geodynamics will retain its identity as the new Commission 3, but sub-entities will change. Geodynamics Section activities within the Commissions, Special Commissions, the Joint Working Group, and the International Services are described in individual reports below. In addition to the work described below, Section V President Wilson headed the Scientific Organizing Committee of the Seventh International Congress of Earth Sciences, in Santiago Chile, October 2002, sponsored by IAG. Additional information is contained in the report of Commission XIV below. Section V Vice President Veronique Dehant chaired the joint IAU-IUGG working group on Precession and Nutation, overseeing development of new precession-nutation models, and accounting for the non-rigid nature of the Earth and influences of geophysical fluids. These studies have been important in implementing fundamental changes in astrometry and modern space geodesy. As discussed in the International Earth Rotation Service (IERS) report below, the IERS began implementing new IAU resolutions that fundamentally redefine the Celestial Reference System (CRS) to take full advantage of precision available with modern VLBI. The new precession-nutation models are an important element of high precision geodesy that enables the study of geodynamics. As the IAG representative to the IERS Directing Board, Section V President Wilson participated in a number of IERS activities, including two workshops related to the implementation of the new CRS. Wilson has also participated in the IERS Global Geophysical Fluids Center work of the IERS, which, as noted below, will become a major element of IAG and Geodynamics Commission work in the future.
The executive officers of Section V (Wilson and Dehant) participated in the effort to map the old IAG onto the new structure adopted in Budapest IAG assembly, in 2001. This new structure places the IERS and other services on the same level as Commissions. This is important because in many ways the services like IERS have become leaders in setting geodetic practice. For example, the IERS conventions document forms, in effect, the geodetic standard in use today. Under the new IAG structure, it should be easier to coordinate activities of the services with those of the IAG, by establishing joint review of such standards and conventions. For example, the IAG plans an inter-commission committee on geodetic standards for this purpose, to carry on the work of Special Commission 3 (reported below). An example of inconsistency between IAG resolutions and the IERS conventions is the treatment of the zero-frequency terms in the Earth tides. This sort of conflict can be reconciled over the next few years in this new inter-commission committee on geodetic standards.
Under the new IAG structure, the initial proposal is for Geodynamics (new Commission 3) to include three main subcommissions. One is concerned with crustal deformation, the second with the impact of geophysical fluids on geodetic observations, and the third with earth tides. A subcommission on crustal deformation would be expected to transfer many of the activities under the current Commission XIV, as reported below, though some changes are possible. The subcommission on Earth tides would also be expected to continue the work of the current Earth Tide Commission, as reported below. The new activity, geophysical fluids, is an outgrowth of Special Commission 8 (Sea Level and Ice Sheets, reported below), the IAPSO/IAG special study group on non-tidal ocean effects (reported below), and several special study groups on atmospheric and related influences on earth rotation that were active over the past period of over a decade. Redistribution of air and water dominate the changes in Earth rotation and gravity field at many time scales (days to decades). This subcommission will link the new Geodynamics Commission 3 to other commissions, and services. Notably, the International Earth Rotation Service (IERS) has a geophysical fluids center, and that activity should form the core of the new geophysical fluids sub-commission. The bigger role of services in IAG, especially of IGS as the biggest player in crustal deformation, should strengthen the activities related to crustal deformation. There are many other opportunities for the future under the new IAG. These are likely to grow out of IAG projects, in combination with a multitude of new measurements available, including satellite-based gravity, ice sheet heights from laser altimetery, as well as the expanding geodetic networks of permanent GPS stations.

2. Commission V: Earth Tides (1999-2003)
Shuzo Takemoto, President 

The 14th International Symposium on Earth Tides (ETS2000) 

The ETS2000 was held in Mizusawa, Iwate, Japan from August 28 to September 1, 2000. The Symposium sessions were: Tidal instrumentation; Results of ground based observations; Tidal observations using space techniques; Modeling of solid earth tides and related problems; Atmospheric and oceanic loading effects; Data processing; Superconducting gravimeters; Tidal studies in tectonic active regions; Tides on planet. 138 participants (including 10 accompanying persons) from 21 countries reported fully on their results of continuing researches on Earth tides and thus contributed to the progress of further research of Earth and Planetary Tides. Proceedings of scientific papers were published in the Journal of the Geodetic Society of Japan Vol. 47, No. 1, 2001. Other Reports on the ETS2000 including the list of participants were published in BIM (BULLETIN D'INFORMATIONS MAREES TERRESTRES). Next International Symposium on Earth TidesThe next (15th) International Symposium on Earth Tides will be held in Ottawa, Canada from 2-6 August 2004.
The following 11 resolutions were adopted at the closing session of the ETS2000.

1./ Recognising the importance of the observation of tidal effects and of the determination of tidal parameters by space geodetic techniques, the ETC recommends to continue this observational effort; to compare the results obtained by different space geodetic techniques between each other and with the results of ground based tidal measurements.

2./ Recognising the importance of the new international services on space geodetic techniques, the ETC recommends that WG6 establishes or intensifies the cooperation with the analysis coordinators of these international services concerning the tidal modelling.

3./ Considering the new fields of tidal research in lunar and planetary geodesy, the ETC recommends that the tidal community should take an active part in space missions related to lunar and planetary geodesy; requests a proper archiving of the data and metadata acquired during those missions and normal access to the world-wide geodetic community.

4./ Considering the increasing interest of the tidal community to lunar and planetary researches, the ETC recommends that a session on tides on the planets should be included in the future earth tides symposia.

5./ Recognising the importance of a global Earth coverage with superconducting gravimeters

for the study of weak geophysical signals; for the determination of the liquid core resonance parameters; for the study of the polar motion effects on gravity; for the intercomparison of the load vectors derived from recent ocean tides models; for the study of global and regional gravity changes to validate the results of the dedicated satellite missions, the ETC recommends to extend the GGP observation period for an additional 6 year period starting July 2003, to maintain the existing sites and to encourage the installation of new GGP stations especially in the Southern hemisphere and in polar regions.

6./ Recognising the fact that presently the calibration of the superconducting gravimeters participating to the world-wide GGP project is not homogeneous, the ETC recommends that systematic calibration campaigns with absolute gravimeters should be planned and realised before the end of the current GGP observation period, through an international cooperative effort.

7./ Recognising the importance to keep in operation several calibration techniques for gravimeters to allow a mutual accuracy control, the ETC recommends that inertial calibration platforms and moving mass calibration devices should continue to be developed or maintained besides more usual calibration methods such as intercomparison with absolute or well-calibrated relative instruments.

8./ Recognising the importance of environmental data for the interpretation of tidal measurements, the ETC recommends to record the following parameters: The barometric pressure, temperature, precipitation, and ground water level. The sampling rate for the recording of environmental parameters should correspond to the sampling rate of the geodynamic data observed. A sufficient resolution and accuracy of the measurements of the environmental parameters should be granted.

Although the difficulties of monitoring soil moisture are recognised, it is recommended to undertake efforts to realize a continuous monitoring of this parameter. The monitoring of wind is also recommended because wind might produce short-period noise as well as long-period modulations; to correct gravity data in long term studies for local (diameter 100km), regional (diameter 2000 km), and global atmospheric pressure signals as all three produce significant effects to develop correction models for gravity, tilt, and strain related to: ground water table variations; snow, rain and soil moisture; stress resulting from temperature variations

9./ Noting the importance for tidal measurements of accurate error estimates and appreciating that such estimates can be made only if the power spectral density of the noise is known, the ETC recommends to show noise spectra as Power Spectral Density expressed in unit 2/ frequency.

10. / On behalf of all participants of the 14th International Symposium on Earth Tides, the ETC thanks the Japanese National Committee for Geodesy, the Science Council of Japan, the Geodetic Society of Japan, the National Astronomical Observatory of Japan, the City of Mizusawa and the Iwate Prefecture for their generous support to the Symposium.

11. / ETC thanks the Local Organising Committee : Masatsugu Ooe (Chairman), Tadehiro Sato (Secretary) , Jiro Segawa (President of Geodetic Society of Japan) and the staff, for their wonderful welcome and their many efforts in making the 14th International Symposium on Earth Tides a great scientific success.

Combined Workshop of ETC-WG6 and GGP

A combined meeting of the ETC Working Group 7: Analysis of Environmental Data for the Interpretation of Gravity Measurements (Chairperson: Corinna Kroner) and the Global Geodynamics Project (Chairperson: David Crossley) was held in Jena, Germany during the period of March 11-15, 2002. The GGP Workshop concentrated on all issues surrounding superconducting gravimetry - instrumentation, site development, data processing, while at the meeting of the WG addressed topics on environmental effects on gravimeter, tilt, strain, and seismometer data. In all, 39 scientists from 16 countries participated. Proceedings of the Combined Workshop of ETC-WG6 and GGP were published in the Bulletin d'Informations des Marees Terrestres (BIM). Vols. 135 and 136, 2002.

Other ETC activities

The new ETC Homepage is http://www-geod.kugi.kyoto-u.ac.jp/iag-etc/ The standard software for the prediction of earth tide phenomena and for the processing of earth tide observations can be downloaded from the Electronic Information Service in ETC HP. The ETC steering committee decided to award the 2nd ETC Medal (ETC Medal 2000) to the late Professor Hans-Georg Wenzel for his outstanding contribution to international cooperation in earth tide research. The ETC awarded the Medal to Ms Marion Wenzel at the Opening Session of ETS2000 on August 28 2000 at Mizusawa, Japan. 


3. Commission XIV: Crustal Deformation
Suzanna Zerbini, Chairperson

General objectives of Commission XIV are: to study 3-D tectonic motions, in active tectonic regions, postglacial rebound and sea-level fluctuations and changes in relation to vertical tectonics along many parts of the coastlines and in relation to environmental fluctuations/changes affecting the geodetic observations; to promote, develop and coordinate international programs related to observations, analysis and data interpretation for the three fields of investigation mentioned above; to promote the development of appropriate models. Commission XIV comprises the following sub-Commissions that have separate sections below: Working group of European Geoscientists for the Establishment of Networks for Earth science Research (WEGENER, Chair: Luisa Bastos, Portugal);Geodetic and Geodynamics programs of the Central European initiative (CEI, Chair Janusz Sledzinski, Poland); Asia-Pacific Space Geodynamics program (APSG, Chair: Ye Shuhua, China); Central and South America (Co-Chairs: Alejandro Gutiérrez, Costa Rica and Rodrigo Barriga, Chile); Africa (Chair: Ludwig Combrink, South Africa); Antarctica (Chair: Alessandro Capra, Italy);
The Commission XIV Bureau met on four occasions: September 21, 2000, San Fernando, Spain; December 17, 2000, San Francisco, USA; March 29, 2001, Nice, France; September 4, 2001, Budapest, Hungary. The last Bureau meeting will take place in July 2003, in Sapporo. During the first meeting, Board membership, Subcommissions organization and Commission activities were discussed and revised. During the subsequent meetings particular attention was given to support and coordinate the work carried out by the Subcommissions. In particular, the Bureau agreed that a major activity of Commission XIV be to support the formation of a global "velocity field" from the combination of regional contributions along the lines of the GPSVEL Project (see WEGENER section of this report). A meeting with the representatives of the Central (including the Caribbean area) and South America Subcommission took place in Miami prior to the December 2000 Bureau meeting in San Francisco. They agreed to survey the relevant activities underway and the resources available. The South America co-chair announced the international Symposium on "Recent Crustal Deformation in South America and Surrounding Areas". This event, supported by the Commission took place in Santiago, Chile, in October 2002 (see South America section of this report). Commission XIV co-sponsored the "Tenth International Symposium on Recent Crustal Movements held in Helsinki, Finland, August 27-31, 2001. There were more than 70 participants in the symposium and 60 presentations covering a wide variety of topics. As a result, a special issue of the Journal of Geodynamics (2003) has been produced containing 13 manuscripts. (Journal of Geodynamics, Special Issue, Crustal Deformation, 2003, vol. 35 N. 4-5). Commission XIV has a web site: http://www.df.unibo.it/commXIV/

WEGENER

During the period 1999-2003 activities were centered on: study of the three-dimensional deformations and gravity along the African-Eurasian plate boundaries and in the adjacent deformation zones in order to contribute to a better understanding of the associated geodynamical processes; monitoring of three-dimensional deformations in a large region centered around Fennoscandia in order to determine the magnitude and extent of the present-day postglacial rebound in that area, thereby extending our knowledge about the viscoelastic properties of the Earth; investigation of height and sea-level variations in order to identify and separate the processes contributing to these variations. Within this period the WEGENER board had the following meetings: Nice, France, April, 27, 2000; San Fernando, Spain, September, 21, 2000; Nice, France, March 26, 2001; Budapest, Hungary, September, 5, 2001; Athens, Greece, June 14, 2002. The main decisions and outcomes of these meetings were: to promote the cooperation of researchers from different geosciences and to strengthen the cooperation with African research groups with enlargement of activities in Africa; to support the GPSVEL project, managed by Geoffrey Blewitt, by promoting the interest and willingness to provide GPS data/solutions within the WEGENER area of interest. As a result, several contributions were sent including solutions from more than 100 stations distributed all over Europe; to support the interest for the realisation of a coordinating Center that could produce "WEGENER solutions" mainly (but not exclusive) from permanent GPS stations and episodic campaigns. There was agreement on the realisation of such a center that should also address issues such as data quality and control and comparison of processes and products. As a first step, the WEGENER Directing Board decided to start the setup of a Data Base Center supporting the generation of a detailed velocity field in the WEGENER study area. This may be seen as a part of the global GPSVEL-Project (Blewitt et al., 2000). This project should start with the collection and combination of GPS and other solutions for coordinates and/or velocity estimates. The center should act in order to guarantee the quality for non-IGS or non-EUREF sites, and should not superimpose, but be complimentary to these Services, by dedicating also special attention to the use of GPS data from episodic campaigns. A second step should be the application of an integrated approach to a selected region where data from different geosciences would be jointly analyzed and interpreted. A first a document entitled “Processing and Submission Guidelines for GPS Solutions to be integrated to a WEGENER Data Base” was prepared by Matthias Becker, Carine Bruyninx and Rui Fernandes to be disseminated among potential contributors. This document contains guidelines and requirements to be followed for submission of solutions to such a Center and concentrates on how GPS campaigns of finite duration can be processed and correctly tied to the ITRF. It was first delivered to the participants of the 11th General Assembly of the WEGENER project that took place in Athens in June 2002. 
During the period 1999-2002, two General Assemblies were organized. One took place at San Fernando, in Spain, in September 2000, hosted by the Real Instituto Y Observatorio de la Armada, and the other one at Vouliagmeni (Athens), in Greece, in June 2002, hosted by the Department of Rural and Surveying Engineering of the National Technical University of Athens. The main themes for these Assemblies were: Geodynamics; Plate tectonics; Integrated observation techniques.Two special issues of international journals were produced. In 2000, the Journal of Geodynamics published a special issue, which compiles six manuscripts resulting from presentations made during the eighth WEGENER general assembly. In 2002, a special issue of Global and Planetary Change was published dealing with height and sea-level variations resulting from the SELF II project. Special support was given to the participation of Colleagues from North African Countries. Pursuing the goal of taking the general Assemblies to different countries of the WEGENER area, and with the purpose to strengthen the cooperation with North Africa, there was a proposal for organizing the next WEGENER Assembly in a North African Country. The Colleagues from Morocco kindly agreed to host the next (12th) general assembly in 2004.


Publications

Blewitt G., Lavallée D., Clarke P.J., Nurutdinov, Holt W.E. Kreemer C., Meertens C.M. Shiver W.S., Stein S., 2000, GPSVEL Project: Towards a Dense Global GPS Velocity Field. In Book of Extended abstracts of the 10th General assembly of the WEGENER Project, San Fernando, Spain, 18-20 September 2000, Boletin ROA, N.3/200, 

Journal of Geodynamics, 2002, Special Issue WEGENER: Observations and Models, vol. 30, N. 3, pp.287-388.

Global and Planetary Change, 2002, Special Issue Sea Level Fluctuations in the Mediterranean: Interactions with Climate Processes, vol. 34, N. 1-2, pp 1-140.


Central European Initiative (CEI)

The first phase of the Project CERGOP was concluded in 1998. The proposal of the second phase of the Project CERGOP-2 "A Multipurpose and Interdisciplinary Sensor Array for Environmental Research in Central Europe (CERGOP-2/Environment)” was selected by the European Commission and will be financially supported during the next three years. Fourteen European countries participate in the second phase and the total number of CERGOP-2 stations is 63. Many CEGRN sites are part of the IGS permanent network. Monitoring GPS CEGRN campaigns were performed in 1999 and 2001 (CERGOP-2). At present, there are 13 study groups covering particular fields of activity supporting the realisation of the Project and, in general, they form the relevant ”workpackages” of the EU Project CERGOP-2/Environment. The CERGOP was an impulse for establishment of the CEGRN Consortium of institutes involved in the realisation of the Project. At the moment 14 European institutions are members of the Consortium.
The UNIGRACE project was launched in 1997 as multipurpose interdisciplinary project and it ended in 2002. It consisted of the establishment of absolute gravity stations covering the area from the Baltic Sea to the Adriatic and the Black Seas. Ten absolute gravity intraplate and seven tide-gauge stations were measured in 12 countries. The gravity observations were carried out by means of five absolute gravimeters from Austria, Finland, France, Germany and Poland. The observation campaigns of the UNIGRACE project were successfully performed in 1998/1999 and 2000/2001. The analyses of the gravity data of the UNIGRACE campaigns indicate that there are considerable changes of the gravity values at some absolute stations. These changes cannot be explained by accounting for the effects of station point displacements, water table level variations, atmospheric effects or other known factors. For this reason, all countries participating in UNIGRACE agreed to continue, as a UNIGRACE follow-up action, the further investigation of the gravity time variations/change at main European stations. 
Section C Working Group on University Education Standards organizes yearly an international educational seminar/workshop or symposium. Cooperation with EGS results in the yearly organization of a symposium on the geodetic and geodynamic programs carried out in the frame of the international cooperation of CEI countries. CEI Section C "Geodesy" and the Subcommission "Geodetic and Geodynamic Programs of the Central European Initiative (CEI)" declares to supply and release to the aims of the Commission XIV the following results: results of the GPS campaigns of CEGRN carried out in the frame of CERGOP; velocity vectors of about 60 sites in Central Europe computed by the CERGOP Processing Centers from the CERGOP, EXTENDED SAGET, EUREF campaigns; information on progress in determination of quasi-geoid in the Tatra Mountains; information on local geodynamic projects realized in Central Europe within the CEI bilateral and multilateral cooperation; information on work progress concerning the 7 CERGOP Study Subgroups CSG.5/i of the CSG.5 "Geotectonic analysis of the region of Central Europe"; information on geotectonic monographs prepared by the CERGOP Study Subgroups CSG.5/i "Geotectonic analysis of the region of Central Europe";information on progress in the realisation of the post-UNIGRACE actions. CEI has a web page at the following address: http://www.gik.pw.edu.pl/igwiag/cei.html


Asia-Pacific Space Geodynamics program (APSG)

The APSG was endorsed by the IAG on its Boulder Meeting in 1995. The early shape of APSG was formed in the First APSG Workshop, May 13-17, 1996 in Shanghai, China. In Tahiti, French Polynesia, May 12-16, the Second APSG Workshop was held. The Third APSG Workshop was held in Tsukuba, Japan, on Oct. 20, 1999, along with the International GPS Symposium. The Fourth APSG Workshop came back to Shanghai on May 14-19, 2001.At the 2001 Shanghai Workshop, the Scientific Working Group N. 4, Gravimetry in the Asia-Pacific Region, was established. Working Groups 1-3 are Geodynamics and Natural Hazards of the Indo-Eurasian Collision, Geodynamics and Natural Hazards of the Western Pacific Region and Impact of Sea Level variations on the Asia-Pacific Region. The Management Board consists of representatives from 9 Countries, namely: Australia, China, France, Germany, Indonesia, Japan, South Korea, Russia and USA. 
Working groups mentioned above have been reported and exchanged on the APSG Workshops and relevant meetings. The Gravimetry Group has a plan to perform Precise Gravity Observation in East Asia with Super Conducting and Absolute Gravimeters (FG-5) in China, Japan and Indonesia, in the 2003-2005 time frame. The Institute of Astronomy, RAS, and the Irkutsk State Technical University organized an APSG International Seminar on Aug. 5-10, 2002, at Irkutsk, Russia. At this seminar, by recognizing the very broad area of GPS applications with mm level of accuracy, a resolution on establishing a Working Group on the Methods of GPS Measurements and Data Processing was accepted. An International Seminar on this topic will be held in early June 2003, in Bishkek, Kyrgyzstan, to exchange and discuss the optimal allocation of observing networks, construction of geodetic monuments, analyses of GPS data, comparison of different software, parameters and processing methods. The seminar is timely and important for precise GPS measurements. During the IAG/IUGG Meeting in Sapporo, a short APSG meeting will be held to discuss the results in recent years and plans for the future. The 5th APSG Workshop will be held in Hong Kong in 2004.The APSG Subcommission has a Web site: http://center.shao.ac.cn/APSG and can be contacted at the following e-mail: apsg@center.shao.ac.cn.

Central America
The activities in South America were mainly concerned with the establishment of RONMAC (Red de Observacion de Nivel del Mar para America Central) in Central America. The RONMAC project has been devised by the U.S. Government in direct response to the impact of Hurricane Mitch on four Central American countries: El Salvador, Guatemala, Honduras, and Nicaragua. Participating Agencies are: United States Agency for International Development (USAID), Funding Agency ; Center for Operational Oceanographic Products and Services, National Oceanic and Atmospheric Administration (CO-OPS/NOAA), of the US Department of Commerce, as Administrating Agency ; Unit for Sustainable Development and Environment of the Organization of American States (OAS/USDE), as Executing Agency; Regional Committee for Water Resources (CRRH), as Regional Coordinating Agency; National agencies in El Salvador, Guatemala, Honduras, and Nicaragua, as direct counterparts and beneficiaries of the RONMAC project .
The main objectives of this project are: a) support development and improvement of the geodetic framework of Central America; b) provide basic meteorological data to national and regional agencies; c) sea-level monitoring and establishment of long-term means sea level data series. At present, there are 11 operational RONMAC stations in Central America: Panama (2), Nicaragua (2), Honduras (1), El Salvador (3), Guatemala (2), Belize (1). There are also five satellite transmitting meteo-marine stations in Costa Rica to be upgraded as RONMAC type. LABCODAT has been established as a data quality control and spare parts center in Costa Rica. Two training courses have been offered to Central American Meteorology and Geophysics technicians with the aim of providing them with the tools for appropriately operate and maintain the stations. Periodic refreshing courses will be conducted in the future.RONMAC Web site (http://www.oas.org/ronmac/) where the user may download the data every three hours is available. Annual tide tables have been included in this site. It is expected this year the inclusion of a larger number of local predictions as well as periodic sea level and SST monthly control maps.

South America
In October 2002, an IAG international Symposium entitled "Recent Crustal Deformation in South America and Surrounding Areas", took place in Santiago de Chile. It was organized in 7 sessions, in which 41 oral and 25 poster presentations were given. Scientists from several countries attended. This event gave also the opportunity to south American geodesy practitioners and students to acquire new knowledge and experience from major experts in the field. In parallel with the above-mentioned Symposium, a meeting dedicated to SIRGAS was held. The Geocentric Reference System for South America is intended to establish and maintain a reference network, together with defining a geocentric Datum. At this meeting, the current situation in each of the countries involved regarding the Reference Framework was presented. The main topic was the mission set for each country for the vertical control of data covering the continent.The presence in Chile at Conception of the Transportable Integrated Geodetic Observatory (TIGO) is of major importance because of the increased interest that this generates for geodetic research on the continent. This is, in fact, the first and only fiducial geodetic station operating in South America.

Africa
The Hartebeesthoek Radio Astronomy Observatory (HartRAO) has in the past four years developed its Space Geodesy Programme to the extent that it has become one of five fiducial geodetic installations in the world. The three major space geodetic techniques; VLBI, SLR and GPS are supported. A DORIS system is also collocated with these three systems HartRAO joined the TIGA pilot project of the IGS as regional datacenter and associate analysis center, in addition to being an IGS regional data center. Two GPS systems have been collocated with tide gauges, a further similar system is in progress. To support densification of the ITRF and the GPSVEL project, a total of seven permanent GPS systems have been installed, two of which are in other countries (Botswana and Zambia). A project has been established to equip each of the 14 Southern Africa Development Community (SADC) countries with at least one permanent GPS system. This will contribute greatly towards studies of the East African Rift system, evaluation of the African plate motion as consisting of the Nubian and Somalian plates, and will facilitate the conversion of the SADC region from obsolete datums (e.g. Clark 1880) to ITRF. Several projects have been initiated to further the study of crustal dynamics. Analysis of data and development of reduction techniques is showing great promise to determine vertical crustal motion due to earth tide effects as determined by GPS. This will allow calibration of gravity changes due to earth tide effects at installations such as superconducting gravimeters, and the longer term component could be used to calibrate satellite (e.g. CHAMP) orbits. HartRAO has collaborated with several institutions to develop space geodetic applications in the region, especially for geodynamics. Future plans include the conversion of MOBLAS6 to a Lunar Laser Ranging capability and the replacement of MOBLAS6 with an SLR2000 system.

Antarctica
The sub-commission promoted activities contributing to the study of crustal deformation processes in Antarctica and enhanced the co-operation with the GIANT (Geodetic Infrastructure of Antarctica) program of SCAR (Scientific Committee on Antarctic Research) WGGGI (Working Group on Geodesy and Geographic Information). The geodynamics studies were performed in close collaboration with the members of the SCAR Antarctic Neotectonics Group of Specialists (ANTEC), an interdisciplinary group empanelled to improve the understanding of the unique character of the neotectonics regime of the Antarctic plate.The main activities focused on: periodic measurement campaigns for crustal deformation monitoring continued in 2001, 2002 and 2003 as well as field campaigns by GPS networks and GPS permanent trackers. At continental level, observations were performed within the SCAR GPS Epoch (Dietrich et al., 2001) and, at regional level, within VLNDEF (Victoria Land Network for DEFormation control) and TAMDEF (Trans Antarctic Mountain DEFormation control). These latter ones were carried out by the Italian Geodesy Project (Capra et al., 2001) and by the NSF scientific Project (Hothem et al. 02).Development of permanent geodetic observatories (GPS, DORIS, VLBI, tide gauges, absolute and cryogenic gravimetry) and the applications of collocation techniques. Integration of local and regional networks was encouraged, particularly by the data processing for international reference frame definition. AUSLIG (Australia) started the TIGA Project, a GPS Tide Gauge benchmark monitoring Pilot Project, regarding the whole Antarctica continent.


Publications

Capra A., Gandolfi S., Mancini F.,Sarti P., Vittuari L.(2001) VLNDEF project: geodetic contribution to geodynamics study of Victoria land, Antarctica. Proceedings of Gravity, Geoid and Geodynamics GG2000 IAG symposium, Banff, Alberta, Canada, July 2000 ,pp. 379-385.

Capra A., Gandolfi S., Mancini F.,Sarti P., Vittuari L . (2001) “VLNDEF project for crustal deformation control of northern Victoria land”. AGS ’01 (Antarctic Geodesy Symposium), St.Petersburg, Luglio 2001, SCAR Report N.21, pp.8-10, January 2002.

Dietrich R., Dach R., Engelhardt G., Ihde J., Korth W., Kutterer H.J., Lindner K., Mayer M., Menge F., Miller H., Muller C., Niemeier W., Perlt J., Pohl M., Salbach H., Schenke H.W., Scohne T., Seeber G., Veit A., Volksen C. (2001) “ITRF coordinates and plate velocities from repeated GPS campaigns in Anatrctica – an analysis based on ndifferent individual solutions”. Journal of Geodesy,74:756-766.

Hothem L., Wilson T., Willis M. (2002) “TAMDEF. GPS measurements of bedrock crustal deformations. Reports of AGS 02 Symposium, Wellington, N.Z. http://www.scar-ggi.org.au/geodesy/ags02/.



4. Special Commission 3: Fundamental Parameters
Erwin Groten, Chairman

Introduction
When present accuracies of the order of ± 10-9 or ± 10-10 (as applied in VLBI = Very Long Baseline Interferometry) are considered, fully relativistic reference frames have to be applied as recently adopted for ICRS (= International Celestial Reference Systems) by IAU (= Intern. Astron. Union) at its General Assembly in Manchester (Groten, 2000a, b). Clear distinction between defining, primary and derived constants is necessary (Kinoshita, 1994). Corresponding updated terrestrial reference systems have been considered by modifying ITRS (= Intern. Terrestrial Reference System) but such modifications have not yet been adopted by IAG or IUGG (= Intern. Union of Geodesy and Geophysics). Ellipsoidal Somigliana-type reference systems of various kind were proposed by Ardalan, Grafarend and others (Grafarend and Ardalan, 2001) in terms of GDR 2000 etc but did not yet replace officially the IAG system GRS 80 (= Geodetic Reference System). See also the Piezetti-type solution by Bursa et al. (2002b) leading to a semi-major axis of a = 6 378 136,6 ± 0.06) m. WGS 84 in its updated form of 1997 (= World Geodetic System of NIMA), where NIMA is the National Imagery and Mapping Agency of the USA, suffers from the fact that the basic four parameters are derived from almost one and the same satellite system so that they are no longer mutually independent. This can lead to inconsistencies. With the new developments in high-precision clocks (in terms of atomic interferometry with cooled atoms) reaching accuracies better than ± 10-14 a new era may start where geodesy contributes stronger to fundamental physics and global high-precision navigation (Heitz, 2001). Kopeikin and Fomalont (www.nature.com/nsu/030106/030106-8.html) recently came to the somewhat controversial and premature conclusion that gravity propagates at about the speed of light leading to a kind of stepwise reconciliation between relativistic and quantum concepts. The main advantage for time measurements in the new system is the linear relation of UT 1 to the new “earth rotation angle”, which replaces Greenwich Apparent Sidereal Time in the future. 

Time variations
Corresponding to the desired “sub-microarc-second” accuracy of ICRS the need for updated geodetic reference systems is obvious. Also in view of temporal variations of the surface the structure and the dimensions of solid and fluid parts of the earth a dynamic global vertical Datum all over the earth, is needed. With modern altimetry and ongoing tide gauge networks, such as GLOSS, WOCE, TIGA etc., the altimetric-gravimetric boundary value problem has to be solved even if it is considered as an “improperly posed problem” in Hadamard’s sense. Pioneer work by Bursa, Grafarend, Kakkuri and others (Bursa et al., 2002a) demonstrate the possibility; even though the accuracy is not yet as good as demanded; for details see (Ardalan and Grafarend, 2001). However, with new altimetric satellites, such as JASON-1, accuracies can be improved. As two thirds of the earth’s surface are ocean, the monitoring of temporal variations is mandatory. As far as tectonic activity on land is concerned, Japan with its permanent GPS-network has demonstrated its abilities to establish very effective monitoring systems. Besides ITRF or, in scaled form, ITRS there is a large number of densifications and other monitoring permanent space geodesy networks so that sufficient information on dislocations and exact coordinates in well defined geocentric reference frames will be available. With the new system, ICRS, IAU also adopted a new time system which is related to a conventional potential value at the geoid (= W°) so that astronomical time systems demonstrate a tendency to go away from the earth, as was already demonstrated by the fact that ICRF is no longer referred to precession and nutation parameters of the earth. In determinations of W° the error inherent in the terrestrial gravitational constant GM is almost negligible since new values of GM (Pavlis, 2002) reconfirm previous determinations. Moreover, it is interesting to realize that the numerical value of W° is less affected by errors in the volume of the geoid than the shape of the level surface of W°, i.e. the absolute geoid (Groten, 2001). Moreover, Wo does not depend on specific permanent tidal regimes. IAG needs the new parameter set in applications of the new formula of precession and nutation as recently adopted by IAU. A typical case is the new value of dynamic flattening H = (C-(A+B)/2)/C where A, B and C are the principal moments of inertia of the earth. It also takes profit from the newly adopted definition and implementation of the “intermediate pole” as a substitute of CEP (= Celestial Ephemeris Pole) and the new NRO (= non-rotating origin) according to Prof. Guinot, replacing the traditional equinox; it is denoted by CEO (= Celestial Ephemeris Origin). Thus we come close to a non-rotating quasi-inertial system in space in the Newtonian sense. With time systems (such as GPS-time, GALILEOSAT-time) we need time and coordinates clearly related to different reference frames, where in view of potential differences general relativistic, and in view of relative motions special relativistic reductions need to be applied. In (Brumberg and Groten, 2001, 2002) several specific aspects have been considered and inconsistencies in the new system were pointed out. Still free wobbles are not yet fully implemented in the definition of the new reference frame. The temporal variations of fundamental contants, such as W0, in terms of dW0/dt º 0 etc., are today as important as the constants themselves or even more important in view of practical problems inherent in the determination of W0 etc. This is relevant in view of consistent reference systems.

Special Geodetic Aspects
With VLBI-measurements, Quasars and other distant sources are observed. With new navigation systems, as in case of EGNOS, geostationary satellites are being used. Thus we have a variety of high-precision observations of macrocosmic type to be carefully processed. Targets are located in celestial and observers are in terrestrial frames. Contrary, in superconducting gravimetry, in tidal or tectonic extensometry and in case of tilt observations typical microcosmic observations are being carried out which are basically related with the aforementioned accuracies to microcosmic dimensions where quantum mechanics, Brownian motion and Heisenberg’s uncertainly relation become relevant. Both types of observations have to be combined and to be integrated into similar systems, where the aforementioned work such as the one by Kopeikin et al. becomes relevant. 
Nevertheless, the mathematical background within potential theory of physical geodesy and conservative forces, where loss of energy vanishes, is relatively simple. But with temporal changes of longer period, as in case of El Nino etc., the role of the atmosphere, on the one hand becomes more import, when mass and energy exchange between oceans and atmosphere needs to be taken into account. On the other hand, the attractive effect of the atmosphere (approximated by a radially stratified homeoid) in space (at satellite altitude) and at various elevations of the surface of the earth varies with time and locations. With CHAMP tomography, atmospheric sounding and limb studies giving temporal variations of the structure of the atmosphere associated results become more important, also for practical applications such as climatic research and weather forecast.. Non-gravitative effects in the atmosphere as well as friction and dissipation in the deeper earth interior put higher demands on mathematical treatment. Thus, viscosity and anelasticity considerations down to the non-hydrostatic CMB (= Core-Mantle-Boundary) and even deeper down to the inner core where Slichter-modes become now relevant, lead to “improperly posed” problems as discussed above which are, however, more complicated than in case of the geodetic BVPs (= Boundary Value Problems).

Practical Aspects
The Subcommission of SC-3 of Prof. J. Rueger (UNSW) which models the atmosphere, has great impact on telemetric and telematic needs in improving refractive models. On the other hand, the use of various tidal results in order to solve the BVP and determine parameters, such as the quality factor, Q, characterizing the distribution of anelasticity in the frequency and space domains, has substantial importance for SEDI (Study of Earth’s Deep Interior). It seems that VLBI-observations yielding Q = 30 000 (for free core nutation frequency) are more accurate and less perturbed by loading and other atmosphere perturbations than superconducting gravimetry. But both techniques now yield Q = 30 000. The difference with respect to other observational results, such as wobble-effective Q etc., do no longer pose problems, as the latter are not as accurate as VLBI and super-conducting gravimetry. Long-period variations related to free-core-nutation represent, at this time, the most reliable results anyway. But shorter period variations still need better observations, as in case of ocean tides or Slichter-modes in the inner core. 
Also the long-term averages of dislocation vectors deduced from permanent GPS-stations still pose problems. Iceland is one of the few locations on earth where the oceanic tectonic activities in ridges are observable at the surface and where, consequently, the convective mass transfer from the deeper earth’s interior to the lithosphere can be observed directly at the earth’s surface. Various extensometric, gravimetric, repeat leveling, GPS and tilt observations were carried out there under quite unique climatic limitations. The aforementioned long-term averages of dislocations usually do not reveal the continuous or discontinuous lithospheric motion along and across such faults which can be observed in Iceland and which reveal more detailed information on types of flow of the recently generated lithospheric material.
As far as the global parameters themselves are concerned, it seems that long-time uncertainties of the numerical value of Newton’s Gravitational Constant, G, have been solved by Gundlach’s recent (Gundlach and Merkowitz, 2000) excellent measurements at Seattle. Another recent determination which yields a slightly higher value is by Quinn et al. (2001). Thus G is now much better determined (Mohr and Taylor, 2002). Whereas the original attempts to interpret discrepancies between various determinations of G in terms of "fifth force" effects (and associated considerations of E. Majorana's gravitational absorption and shielding) have lost a lot of their previous interest (Zürn, 2003), recent speculations around the geomagnetic field influences on measurements of G are now under consideration (Mbelek and Lachièze-Rey, 2002). Latitude-dependent discrepancies, real or apparent, are characteristic for modeling processes of terrestrial parameters; Kaula discussed such phenomena in case of Lamé’s constants, Wahr found them in case of tidal parameters; other examples are well known. In so far such a discussion around G is not surprising. New research in view of ongoing G-determinations appears necessary. With the new definition of the earth’s rotation vector, a clear separation is needed in polar motion and UTC (= Universal Time Coordinated) as well as LOD (= Length of Day) measurements between new and earlier observations in view of high-frequency differences between “intermediate” and “celestial” Ephemeris Pole. The temporal changes of obliquity (of the ecliptic) deviating from standard modeling, which were postulated by B. Chao and others, possibly affect climatic variations in addition to the Milankovich’s-concept and other green-house perturbations may also become relevant in the new reference frames. 

The role of gravimetry
The recent determination of the terrestrial gravitational constant
GM = 398 600 441.7) ´ 106m3s-2 by E. Pavlis (2002) and the update of the Newtonian gravitational constant by Gundlach et al. from the University Washington in Seattle leading to a new optimal estimate (Varga, private comm., 2002) of G = (6672.1 ± 30) ´ 10-14m3s-2 kg-1 imply a substantially improved scaling of terrestrial and satellite gravity spaces. Intuitively, I still trust more in a slightly higher G-value such as 6672.6 ´ 10-14 m3s-2 kg-1 held in the past by SC-3. The formal accuracies of recent G-determinations are of the order of ± 10-5 thus indicating the increased quality of observations. Summarizing the present “state of the art” we may conclude that there are basically two slightly different opinions: that the bias is much larger in the older observations so that a weighted average between the recent measurements by Quinn et al. and those by Gundlach and Merkowitz should yield the best estimate of G, leading to a numerical value around G = (6674.1 ± 0.09) ´ 10-14m3s-2 kg-1, would be appropriate; or that the biases are not yet well understood in all observations so that, with appropriate caution, a “current best estimate” should be derived from all available observations until new results are obtained from ongoing experiments.
A third (substantial) revision of previous reports of SC-3 (Groten, 2000a) is related to “secular” and aperiodic global changes of the earth’s shape and its gravity field. We meanwhile well understand that those changes are basically superpositions of solid earth, atmospheric and oceanic effects. This is similar to the interaction of atmospheric, loading, solid earth and oceanic tidal effects in the periodic part of the spectrum. Moreover, in view of relatively short and narrow observation windows, long-periodic tides, such as the 18.6 year period, etc. may interact with “secular” perturbations. GRACE will contribute to a significant step forward. The second zonal harmonic C2 is a good example which is representative for other parameters, too. Cox and Chao (2002) have pointed out that the decrease in C2 is more than compensated since 1998 by a positive Whereas the decrease was supposed to be a consequence of postglacial rebound the surprising change since 1998 could be due to oceanic effects which did not exist in this form and in such a size before 1998. We may hope to detect similar variations and “turn-around” effects in the future for other global parameters of the earth. Therefore, care is necessary in deriving “secular” variation rates from short observation intervals; as far as this example is representative also for other global parameter variations originating from similar complex geophysical origins (superposition), “secular” changes derived in the past may not always be considered as really secular.
It was generally considered a great honor when I was elected at the IUGG General Assembly at Moscow in 1971 to chair the Special Study Group (SSG) of IAG on Special Techniques in Gravimetry in succession of the great Tauno Honkasalo whose outstanding reputation was lateron mistakenly jeopardized by erroneous interpretations (for details see Hipkin, 2002) of the so-called “Honkasalo-term” in permanent tides. Heikkinen did a lot to repair those misinterpretations. Unfortunately, many details of the achievements of gravimetry were never published but rather privately circulated. One reason for that were the well known deficiencies of gravimetry where (1) the well known non-uniqueness of gravimetric interpretations, (2) unmodeled perturbations in instrumental drift and (3) unmodeled effects in differential equations in formulating uplift in post-glacial investigations and isostatic modeling led to a series of controversies. So I refrain here from referring in detail to numerous publications or unpublished results. Moreover, still unexplained features do exist. Deficient static models of isostatic equilibrium, starting from Airy-Heiskanen over Pratt-Hayford and Vening-Meinesz’ lateral isostaty could never fully explain dynamic isostatic behavior, as, e.g. formulated by D. Peltier. Insofar also A. Bjerhammar’s dynamic uplift models never fully explained the actual situation in Fennoscandia and the long series of repeat gravimetry only led to useful results of post-glacial uplift when GPS and similar additional technology became available. Nevertheless, the viscosity deduced from such uplift rates is very valuable in deducing viscosity for the earth mantle. Superconducting gravimetry led to a substantial step forward and gave way to numerous new results in studying the earth’s inner core. Absolute gravimetry, as one of the very few absolute measurement techniques in geodesy, contributed a lot and will still contribute in the future when altimetry together with tide gauge data over oceanic areas and second derivatives of the potential obtained from GRACE, GOCE etc. projects will globally lead to potential differences which need stabilization within the associated integration processes. It is a pity that not all absolute gravity measurements are being published, as those by NIMA (National Imagery and Mapping Agency) and other organizations. This concerns also J. Faller’s results where he used P. Varga’s heavy ring calibration technique to contribute to a new determination of the Newtonian gravitational constant in connection with recent discussions on Fifth Force Experiments. The role of mechanical stationary relative gravimeters was evaluated when W. Zuern emphasized, at that time, the superiority of some of such meters over superconducting gravimeters in the high-frequency domain >1mHz. In so far, still the full spectrum of terrestrial gravimetry is appreciated in spite of its inferiority to satellite, mainly LEO-type satellites, in global studies. Thus B. Chao’s investigations on the contribution of numerous big reservoirs on earth to changes of moments of inertia of the earth and his evaluations of periodic atmospheric (and oceanic) effects in time variability of low degree harmonics in the earth’s gravitational field could never sufficiently be verified by terrestrial gravimetric experiments. The study of atmospheric gravitational fields variations was limited to very local and also loading effects. Particularly, GRACE-data will substantially improve our knowledge previously deduced from more precise information on low degree (and low order) gravity field harmonics variability. However, there is a variety of perturbing effects which needs more detailed consideration. One of them is ocean floor pressure variation. This effect acts over large parts of the time spectrum. It also affects other geodetic observations, such as polar motion. Ocean bottom effects were over long time, almost ignored. Regional Earth’s surface (crustal) and upper mantle variations will still take profit from more detailed gravitational information using stationary terrestrial gravimetry. Aperiodic earthquake precursors as detected by us in the seismic belt of China at Lanzhou still need further detailed investigations, as too many unmodeled effects still exist and too many possible candidates can be associated with the observed anomalies in the gravimetric records (Groten, 1996). Anyway, gravimetry (in moving vehicles) in combination with precise positioning and determinations of temporal changes of position now enables improved interpretations which sufferred in the past from local equivalence of inertial and gravitational accelerations. But still numerous unmodeled instrumental effects need explanations in terrestrial gravimetry. A. Kiviniemi was one of the pioneers in that respect but even he could not explain, e.g., surprising tares and jumps in mechanical gravimetric records observed in the Rhinegraben after heavy rainfall etc. which might be indications of rapid gravity changes of the order of some tens of microgals.

Concerning mantle anelasticity and tidal friction:
Already more than a decade ago, at the Intern. Earth Tide Symposium of IAG at Helsinki in 1989, I had formulated the assumption that anelasticity as deduced from tidal gravimetry is often overestimated. Zschau found in 1986 from Chandler wobble data Q @ 270 at principal tidal frequencies such as M2. Ray (2002) attributes only about 5 percent of total tidal friction to solid earth dissipation; he found for M2 a value of about Q = 280 (Ray et al. 2001). In a detailed investigation he attributes for the principal tidal part 2.4 terawatts to the ocean, 0.1 terawatts to the solid earth and the rest of the total planetary dissipation of 2.536 terawatts to the atmosphere. Whereas 1.7 terawatts is attributed to shallow seas, about 0.7 terawatts is attributed to the deep ocean. As original Chandler wobble data had indicated Q @ 100 and less, the extremely low anelasticity for the FCN-period could appear as a clear indication of the significant difference between strong ocean and low solid friction.
Concerning specific modes:
Concerning the free core nutation (FCN) period specifically, it had been presumed that air pressure perturbations may strongly perturb results of superconducting gravimetry, leading to a quality factor of the order of Q = 3000 and more. With the classical definition of Q as the ratio of dissipation DE and total energy E over one cycle we get which is free of any mechanism. Meanwhile Q = 30 000, as derived from VLBI-data has been corroborated almost exactly by Prof. Sun Heping and Prof. Sato using all available superconducting gravimetric results after careful reductions (priv. communication, 2002). For the foregoing value there is a significant numerical deviation from the wobble-effective values of Q around 200. Summarizing the present “state of the art” we may conclude that the quality factor Q does slightly depend on frequency and even more on the underlying mechanism of friction and dissipation. The overwhelming part may occur in the ocean, two thirds of it in the shelf areas and some part at the core-mantle boundary (CMB).

Conclusions
In view of a variety of practical needs the improved determination of global “parameters” (not really “constants”) appears important (Adam and Schwarz, 2001). More real-time high precision solutions for navigation etc. are demanded and related precise global needs have to be fulfilled. Temporal changes of harmonic coefficients of the geopotential, besides the low-order zonal harmonics, are quite intricate in their determination but need to be considered up to higher degrees of tesserals and sectorials. Also the different types of earth rotations variability (from sub-diurnal to tidal friction) need additional investigation. This holds also for long-term polar migration and its interpretation in terms of polar cap melting processes. The results found for profiles of mantle viscosity strongly depend on assumptions on CMB-flattening. The results by Molodensky and Groten (2001; 2002) agree, to some extent, quite well with those found earlier by Herring et al. (1986) and others. Nevertheless, the non-hydrostatic CMB-flattening still poses problems and leads to discrepancies in the results. The long-term variations (periods of diurnal period upwards) still perturb a variety of other measurements, as in case of the quality factor Q, and loading and similar effects still need more intensive studies. In contrast, also the sub-diurnal variations of the earth’s spin rate involves still open questions. The aforementioned results obtained from GPS give more qualitative than exact quantitative information, even though valuable information on ocean-atmosphere interaction has been found. Also the origin of El-Nino Southern Oscillation is still an open question. The role of ocean bottom pressure variations and its impact is surprisingly pronounced in various effects. 
Acknowledgement: Very valuable discussions with W. Zürn on various topics are appreciated. 


References

Adam, J. and K.-P. Schwarz (Eds.): Vistas for Geodesy in the New Millennium. IAG Symposia, Vol. 125, Scientific Assembly, Budapest, Hungary, Sept. 2-7, 2001, Springer Verlag, Heidelberg.

Ardalan, A. and E. Grafarend (2001): A new estimate of the time derivative of the Geoid Gauge , presented at the IAG scientific assembly, Budapest, 2-8 September, 2001.

Brumberg, V.A. and E. Groten (2001): A note on Earth’s rotation angular velocity in the general-relativity framework. Journal of Geodesy 75: 673-676.

Brumberg V.A. and E. Groten (2002): IAU resolutions on reference systems and time scales in practice. Astron. Astrophys. 347: 1070-1077.

Bursa, M., E. Groten, St. Kenyon, J. Kouba, K. Radej, V. Vatrt, M. Vojtiskova (2002a): Earth dimension specified by geoidal geopotential. Stud. geophys. geod. 46 (2002), 1-8.

Bursa, M., St. Kenyon, J. Kouba, K. Radej, Z. Sima, V. Vatrt, M. Vojtiskova (2002b): Dimension of the earth’s general ellipsoid. Earth, Moon and Planets 91: 31-41.

Cox, C.M. and B.F. Chao (2002): Detection of a largescale mass redistribution in the terrestrial system since 1998. Science 297, 831-833.

Grafarend, E. and A. Ardalan (2001): Time evolution of a world geodetic datum: IAG Symposia, Vol. 125, Scientific Assembly, Budapest, Hungary, Sept. 2-7, 2001, Springer Verlag, Heidelberg.

Groten, E. (1996): Stationary Gravity Measurement at Lanzhou, Shanghai and Wuhan. Geo- wissenschaften 14, Heft 7-8:313-314.

Groten, E. (2000a): Report of Special Commission 3 of IAG, in: Proc. IAU Coll. 180 “Towards Models and Constants for Sub-Microarcsecond Astrometry (K. J. Johnsten et al., eds.), 337-352, U.S. Naval Obs. Washington.

Groten, E. (2000b): Report on “Fundamental Geodetic Parameters” of IAG-Special Commission 3, IAU-Gen. Ass., Manchester August 2000.

Groten, E. (2001): Do we need a new reference system? IAG Symposia, Vol. 125, Scientific Assembly, Budapest, Hungary, Sept. 2-7, 2001, Springer Verlag, Heidelberg.

Gundlach, J.H. and St. M. Merkowitz (2000): Measurement of Newton’s Constant Using a Torsion Balance with Angular Acceleration Feedback. Physical Review Letter, Vol 85, No. 14, 2869-2872.

Heitz, S. (2001): Gravimeter und Gradiometer. Mitt. Geod. Inst., Rhein. Friedr.-Wilhelms-Univ. Bonn, Nr. 88, 2. verb. Aufl., Bonn.

Herring, T.A., C.R. Gwinn, I.I. Shapiro (1986): Geodesy by radio interferometry: studies of the forced nutation of the Earth. J. Geophys. Res., Vol 91, No B5, 4745-4765.

Hipkin, R.G.: Is there a need for a geodetic datum 2000? Discussion of a “Heiskanen & Moritz’ proposition”. IAG Symposia, Vol. 125, Scientific Assembly, Budapest, Hungary, Sept. 2-7, 2001, Springer Verlag, Heidelberg.

Kinoshita, H. (1994): Is the equatorial radius of the earth a primary constant, a derived constant or a defining constant? Studia Geophys. et Geodaetica, 38/2, 109-116.

Mbelek, J.P. and M. Lachièze-Ray (2002): Possible Evidence from Laboratory Measurements for a Latitude and Longitude Denpendence of G (Manuscript)

Mohr, P.J. and B.N. Taylor (2002): The Fundamental Physical Constants, in: Physics Today, Aug. 2002, 423-430

Molodensky, M.S. and E. Groten (2001): On the upper bound of the liquid core viscosity. Studia geophys. et geodaetica, 45, 1, 12-36.

Molodensky, M.S. and E. Groten (2002): On the models of the lower mantle viscosity consistent with the modern data of core-mantle boundary flattening. Studia geophysica et Geodaetica, vol 46/3, 411-433.

Pavlis, E (2002): Dynamical Determination of Origin and Scale in the Earth System from Satellite Laser Ranging. IAG Symposia, Vol. 125, Scientific Assembly, Budapest, Hungary, Sept. 2-7, 2001, Springer Verlag, Heidelberg, 36-41.

Quinn, T.J.et al (2001): A New Determination of G Using Two Methods, in: The American Physical Society, Vol 87, No.11, Sept. 2001.

Ray, R.D., R.J. Eanes and F.G. Lemoine (2001): Constraints on energy dissipation in the Earth’s body tide from satellite tracking and altimetry. Geophys. J. Int. 144:471-480.

Ray, R. (2002): Tidal friction in the Earth and Ocean, Proc. 89th Journées Luxembourgeoises de Géodynamique, Nov. 12-14, 2001.

Zürn, W. (2003): Fifth form experiments and Newtons's Big G, DGG-Mitt. (in press).



5. Special Commission 8: Sea Level and Ice Sheets
Michael Bevis, Chairman


During the reporting period SC 8 organized the development of a broader working group, called CGPS@TG, to address the technical issues associated with retrofitting of tide gauges with continuous GPS stations. This is a joint working group of IAG (SC 8), IAPSO (CMSLT), the IGS, PSMSL and GLOSS, and has a website at www.soest.hawaii.edu/cgps_tg. We organized an international meeting in Honolulu, which eventually led to the production of a position paper 'Technical Issues and Recommendations Related to the Installation of Continuous GPS stations at Tide Gauges' which was published by Bevis et al. (2002) as well as being reproduced on the CGPS@TG website. This website also contains a number of useful case studies. SC 8/CGPS@TG organized a second and larger international meeting in Toulouse, France in Sept. 2002, which was funded by the IUGG, with additional support from IOC. Most of the funding was used to provide travel support, with priority given to scientists coming from developing countries. This meeting had an associated one-day workshop for people just entering the field (usually ocenaographers with little geodesy background, or geodesists with little background in oceanography and tide gauges). While SC 8 and CGPS@TG have focused mainly in the science driving the positioning of tide gauges, the technical requirements of field work, and the accumulation of metadata, we are not funded to perform operational activities. The most pressing problem in this area was the lack of a suitable GPS data processing stream. Both SC 8 and CGPS@TG worked with IGS to lobby for, and later to support the development of an IGS pilot project (TIGA) that will provide operational support via global geodetic analysis of the data produced by the CGPS@TG community. Finally, the two international meetings were used to introduce oceanographers to relevant developments in geodesy and geophyics, most notably the measurement and modeling of glacial isostatic adjustment (postglacial rebound), and the difficulties and opportunities presented by seasonal signals in the position times series of CGPS stations, both at tide gauges and more generally. The aim of SC8 is an important topic in the Antarctic research community. In cooperation with the SCAR working group on Geodesy and Geographic Information (since 2002 "Standing Scientific Group on Geosciences") the compilation of Antarctic tide gauge data and their reference to the ITRF by GPS fixing made good progress. Furthermore, ground truth activities for the new satellite gravity missions (CHAMP,GRACE) and altimetry missions (ENVISAT,ICESat) have been supported. 

Reference: Bevis, M., W. Scherer, and M. Merrifield (2002) Technical Issues and Recommendations Related to the Installation of Continuous GPS stations at Tide Gauges, Marine geodesy, 25, 87 - 99.



6. Joint Working Group on Geodetic effects of non-tidal oceanic processes
Richard Gross, Chairman

The IAG/IAPSO Joint Working Group (JWG) on Geodetic Effects of Nontidal Oceanic Processes was formed at the XXII General Assembly of the IUGG that was held in Birmingham during July, 1999 for the purpose of: (1) promoting investigations of the effects of nontidal oceanic processes on the Earth’s rotation, deformation, gravitational field, and geocenter; and (2) fostering interactions between the geodetic and oceanographic communities in order to gain greater understanding of these effects. Since it was formed, five meetings-of-opportunity of the JWG have been held: (1) on December 15, 1999 in conjunction with the 1999 Fall Meeting of the AGU that was held in San Francisco, California; (2) on April 27, 2000 in conjunction with the XXV General Assembly of the EGS that was held in Nice, France; (3) on March 29, 2001 in conjunction with the XXVI General Assembly of the EGS that was held in Nice, France; (4) on April 25, 2002 in conjunction with the XXVII General Assembly of the EGS that was held in Nice, France; and (5) on April 7, 2003 in conjunction with the 2003 Joint Assembly of the EGS-AGU-EUG that was held in Nice, France. Summaries of the latter four meetings have been or will soon be published in the IAG Newsletter that appears in the Journal of Geodesy. 
In the last few years a number of exciting developments have occurred in the area of ocean/solid Earth interactions. The importance of oceanic processes to exciting polar motion in general (Furuya & Hamano 1998; Ponte et al. 1998; Johnson et al. 1999; Nastula & Ponte 1999; Chen et al. 2000; Nastula et al. 2000, 2002; Wünsch 2000, 2002a, 2002b; Ponte et al. 2001, 2002; Thomas et al. 2001; Leuliette et al. 2002a; Ponte and Ali 2002) and the Chandler wobble in particular (Celaya et al. 1999; Ponte & Stammer 1999; Gross 2000; Brzezinski & Nastula 2002; Brzezinski et al., 2002b; Gross et al. 2003; Liao et al. 2003) has been demonstrated by applying ocean models to Earth rotation studies. In addition, ocean models have enabled the relatively smaller, but still detectable, influence of oceanic processes on the length-of-day (lod) to be studied (Marcus et al. 1998; Johnson et al. 1999; Chen et al. 2000; Kakuta et al. 2000; Ponte & Stammer 2000; Höpfner 2001; Ponte and Ali 2002; Ponte et al. 2001, 2002). The oceanic torques acting on the solid Earth are also being studied (de Viron et al. 2002; Fujita et al. 2002; Hughes 2002). As global ocean general circulation models continue to improve, and ocean data assimilation systems develop (Ponte et al. 2001), more progress can be expected.
Since the formation of the Joint Working Group, the effect of oceanic mass redistribution on the orbits of satellites (Chen et al. 1999a; Johnson et al. 2001a), the Earth’s gravitational field (Cazenave et al. 1999; Cheng & Tapley 1999; Gruber et al. 2000; Foldvary and Fukuda 2001a, 2001b, 2002; Johnson et al. 2001b; Leuliette et al. 2002b; Wünsch et al. 2002; Reigber et al. 2003), geocenter (Cazenave et al. 1999; Chen et al. 1999b; Bouillé et al. 2000; Johnson et al. 2001b; Crétaux et al. 2002), surface gravity measurements (Sato et al. 2001), and station positions (Mangiarotti et al. 2001) have also been studied. The launch of CHAMP and GRACE will enable even more detailed studies of the influence of the oceans on the Earth’s gravitational field (Wahr et al. 1998). Furthermore, CHAMP and GRACE will directly measure the mass term of the Earth rotation excitation functions (Gross 2001, 2003) as well as fluctuations in ocean-bottom pressure (Ponte 1999). Thus, the next few years should prove as exciting as have the last few years in studying the geodetic effects of nontidal oceanic processes. 
As can be seen in the studies of, for example, the EOPs (Earth Orientation Parameters) and the change in the J2 term of the global gravity field, the oceans play an important role in Earth system dynamics (ESD). Measurements of sea surface height (SSH) variability obtained from satellite altimetry have revolutionized our knowledge of the role of the oceans. For the ESD, the important physical quantity directly affecting its change is the mass redistribution in the oceans, not the change in the SSH itself, because, as is well known, the observed SSH variations from satellite altimeters include as a major part steric changes due to thermal and salinity changes in the oceans which do not produce any gravity effects. By combining the altimeter data with the data obtained from satellite gravity missions such as CHAMP, GRACE and GOCE, we can expect to separate the steric changes and to reveal the true mass transport in the oceans. On the one hand, the validation and calibration of satellite data based on observations on the ground, in the ocean and on the sea floor are important for interpreting the satellite data because the observation data by satellites are a result of integrating over many related phenomena. More over, for the data obtained from the gravity satellites, the effect of air pressure changes on the ocean surface is contaminated by an effect of aliasing. In order to study the mass transport in the oceans and the aliasing effect in the satellite gravity data due to air mass changes on the sea surface, it is recommended to develop network campaign observations with ocean bottom pressure gauges at selected locations in the world oceans. An ocean region where the El Niño effect appears is a candidate for this observation purpose and the network should be designed so that we can detect the mass transport in not only the east-west direction but also the north-south direction. Reports from individual JWG members on their activities are given below:

A. Brzezinski. We studied the nontidal oceanic influences on Earth rotation by using the methods which had been earlier developed and applied for estimating atmospheric effects (Brzezinski et al. 2002a). We focused our attention on the excitation of the 14-month free Chandler wobble. By using an 11-year time series of the ocean angular momentum (OAM) we concluded (Brzezinski and Nastula 2002) that within the limits of accuracy the coupled system atmosphere/ocean fully explains the observed Chandler wobble during the period 1985-1996. Similar study using a 50-year OAM series (Brzezinski et al. 2002b) yielded less promising results, which could be attributed to the differences in the underlying ocean circulation models. Our first attempt to estimate the nontidal oceanic contribution to nutation (Petrov et al. 1999) showed that the OAM series produced so far were still not adequate for studying the diurnal and subdiurnal effects. The most important tasks for the future within the subject of the oceanic excitation of Earth rotation were pointed out in a review paper (Brzezinski 2003). 

B. Chao. The Space Geodesy research group at the NASA Goddard Space Flight Center made numerous studies on oceans' roles in changing the Earth's rotation and low-degree gravity field. In addition to the global effects, three specific ocean basins have been targeted: North Atlantic (in association with the North Atlantic Oscillation), Mediterranean Sea (a joint project with scientists from the University of Alicante, Spain), and the extratropical Pacific (in searching for the causes of the 1998 anomaly in the Earth's J2 series found in satellite-laser-ranging data). Three main types of data are examined: (1) TOPEX/Poseidon altimetry data, (2) sea-surface temperature data, (3) ocean general circulation model output. Extensive use of the Empirical Orthogonal Function / Principal Component numerical technique has been made. Progress has been reported at various international meetings, including the AGU, EGS, and T/P SWT. Publications include Chao and Zhou (1999), Johnson et al. (1999, 2001b), Chen et al. (2000), O’Connor et al. (2000), Chao et al. (2001), and Fujita et al. (2002).

T. Johnson. The U.S. Naval Observatory in an effort to improve its predictions of UT1-UTC examined the usefulness of introducing atmospheric angular momentum data into the EOP combination and prediction process. The study showed that the atmosphere could not account for all of the variability on time scales ranging from six days to 15 days. Using the Parallel Ocean Climate Model (POCM), we determined that this variability was the result of non-tidal ocean variability (Johnson et al. 2003). These results along with the results of earlier studies (Ponte et al. 1998; Johnson 1998; Johnson et al. 1999; Ponte et al. 2002) indicate that the ocean appears to excite variations in Earth's rotation on time scales ranging from a few days to a few years. In another study, Johnson et al. (2001b) indicated that non-tidal oceanic variability could account for some of the perturbations in the orbits of the Lageos I and Lageos II satellites on timescales of several days to a few years that result from temporal variations in the Earth's gravitational field but that are unexplained by the atmosphere. Furthermore, it was shown by Johnson et al. (2001a) that the effects of oceanic variability could be observed in the orbit perturbations of GPS satellites. We have also shown that the intermediate and lower layers of global ocean circulation models appear to require much more than 30 years to spin up (Johnson 1999). However, the application of sea level adjustments, also known as the Greatbatch correction, effectively removes most of this effect as well as the effects of the Boussinesq approximation. These results indicate that the examination of trends in momentum would be a better test of model spin-up than the use of kinetic energy.

J. Nastula. It was shown that oceanic excitation, when added to atmospheric excitation, leads to substantial improvements in the agreement with observed polar motion excitation at seasonal and intraseasonal periods (Nastula and Ponte 1999). It was also shown by Brzezinski and Nastula (2002) that variations of the angular momentum of the coupled atmosphere/ocean system (with a little bit higher contribution from the ocean source) can explain within the error limits the observed Chandler motion during the 1985-1996 interval. In more recent studies, Brzezinski et al. (2002b) extended the analysis of Brzezinski and Nastula (2002) by using a 50-year time series of OAM estimated by Ponte (2000, personal communication). The obtained estimate of the oceanic excitation power, 18.1 mas2/ cpy, is in good agreement with the residual excitation derived from a simultaneous use of polar motion and atmospheric angular momentum data, 20.3 mas2/ cpy. Regional patterns of atmospheric and oceanic excitation were analysed separately and compared with each other (Nastula et al. 2000, 2002). The results confirm recent findings that oceans supplement the atmosphere as an important source of polar motion excitation. Analysis of regional AAM and OAM signals were performed for monthly and longer periods (Nastula et al. 2000) and for periods shorter than 10 days (Nastula et al. 2002). The influence of specific geographic areas on polar motion excitation was discovered. Regional characteristics of short period excitation of polar motion are generally in agreement with those obtained from analyses performed for signals at monthly and longer periods. The AAM and OAM signals associated with pressure terms were found to be of the same order of magnitude, while signals associated with winds were substantially larger then those associated with ocean currents. The strongest polar motion excitation due to variability of atmospheric pressure, oceanic pressure and wind terms is connected with some specific areas over northern and southern mid-latitudes. The spatial pattern of pressure plus inverted barometer (IB) term is dominated, however, by maxima over land areas with Eurasia being especially important. Oceanic excitation due to currents is strong in the North Pacific and the Southern Oceans. Variability in oceanic bottom pressure tends to be large in mid and high latitude regions. 

R. Ponte. Ponte et al. (2002) addressed the influence of climate variability on ocean angular momentum (OAM). Possible anthropogenically induced signals included trends and changes in the seasonal cycle of OAM, but their effects on Earth rotation were relatively weak. In contrast, OAM signals related to natural climate variability were found to be important sources of excitation, particularly for the annual, Chandler, and Markowitz wobbles. Ponte and Ali (2002) demonstrated the role of OAM signals for excitation of sub-weekly polar motion and length-of-day variations and the importance of deviations from an inverted barometer response to atmospheric pressure at these rapid time scales. Uncertainties in the atmospheric pressure fields remain a problem in determining those signals (Ponte and Ray 2002; Ponte and Dorandeu 2003). Similar uncertainties in seasonal wind stress torques over the ocean, which affect OAM and the planetary angular momentum budget, were discussed in Ponte et al. (2003). 

T. Sato. Differential GPS observations were conducted on the fast ice in Lutzow-Holm Bay, Antarctica (Aoki et al. 2000). The vertical displacement, was clearly detected. Tidal variation derived from GPS showed good agreement with those from pressure gauges. GPS measurements of the vertical displacement of fast ice near Syowa Station, Antarctica, were conducted between April and December 1998 (Aoki et al. 2002). The GPS derived sea level, combined with observed sea ice thickness, supported a conventional bottom pressure gauge result with an RMS error of 0.007 m. Coherent sea level variations were clearly detected for five coastal tide gauge data around Antarctica on intraseasonal time scales (Aoki 2002). The coherent variations had significant negative correlations with an index of the atmospheric annular mode variation (Antarctic Oscillation). Important new findings from 14 months of observations of ocean bottom pressure variations in the southeastern Pacific are reported by Fujimoto et al. (2003). One is a pressure increase starting in December 1997 at almost the same time as the termination of the 1997-98 El Niño. It is also coincident with a remarkable change in the J2 term of the Earth's gravity field. These results suggest that the El Niño might have brought about mass redistribution in the eastern Pacific Ocean. The other feature in the observations is a local pressure variation across the spreading axis of the ultra-fast spreading southern East Pacific Rise. It is estimated that the seafloor near the spreading axis was depressed at a rate of about 20 mm/month. The gravity effects of sea surface height (SSH) variations were studied by Fukuda et al. (1999). They applied the EOF (Empirical Orthogonal Function) analysis to both SSH data and induced gravity changes and showed that one of the EOF components was strongly correlated with ENSO (El Niño/Southern Oscillation) like SSH variations. Based on the POCM (Parallel Ocean Climate Model, Stammer et al. 1996) and the TOPEX/POSEIDON (T/P) altimeter, the effects of SSH variations on gravity observations were estimated (Sato et al. 2001). The thermal steric component of SSH variations was estimated by assuming a simple linear relationship between the time variations in the SSH and SST fields. The predicted gravity changes at the three observation sites (i.e. Esashi in Japan, Canberra in Australia and Syowa Station in Antarctica) were compared with the actual data obtained from the superconducting gravimeters installed at these three sites. We have also tried to investigate the effects of SSH on the gravity observations in other frequency bands. Our computations suggest that ENSO-like ocean oscillations contribute 2 to 3 microgals peak-to-peak amplitude to gravity variations in the equatorial Pacific at the maximum.

REFERENCES

Aoki, S., Coherent sealevel response to the Antarctic Oscillation, Geophys. Res. Lett., 29(12), doi:10.1029/2002GL015733, 2002. 

Aoki, S., T. Ozawa, K. Doi, and K. Shibuya, GPS observation of the sea level variation in Lutzow-Holm Bay, Antarctica, Geophys. Res. Lett., 27, 2285-2288, 2000. 

Aoki, S., K. Shibuya, A. Masuyama, T. Ozawa, and K. Doi, Evaluation of seasonal sea level variation at Syowa Station, Antarctica, using GPS observations, J. Oceanogr., 58, 519-523, 2002. 

Bouillé, F, A. Cazenave, J. M. Lemoine, and J. F. Crétaux, Geocentre motion from the DORIS space system and laser data to the Lageos satellites: Comparison with surface loading data, Geophys. J. Int., 143, 71-82, 2000.

Brzezinski, A., Oceanic excitation of polar motion and nutation: An overview, in IERS Technical Note 30: Proc. IERS Workshop on Combination Research and Global Geophysical Fluids, edited by B. Richter, in press, Bundesamts für Kartographie und Geodäsie, Frankfurt am Main, Germany, 2003.

Brzezinski, A., and J. Nastula, Oceanic excitation of the Chandle wobble, Adv. Space Res., 30, 195-200, 2002.

Brzezinski, A., Ch. Bizouard, and S. Petrov, Influence of the atmosphere on Earth rotation: What new can be learned from the recent atmospheric angular momentum estimates?, Surv. Geophysics, 23, 33-69, 2002a.

Brzezinski, A., J. Nastula, and R. M. Ponte, Oceanic excitation of the Chandler wobble using a 50-year time series of ocean angular momentum, in Vistas for Geodesy in the New Millennium, edited by J. Adám and K.-P. Schwarz, pp. 434-439, IAG Symposia vol. 125, Springer-Verlag, New York, 2002b.

Cazenave, A., F. Mercier, F. Bouille, and J. M. Lemoine, Global-scale interactions between the solid Earth and its fluid envelopes at the seasonal time scale, Earth Planet. Science Lett., 171, 549-559, 1999.

Celaya, M. A., J. M. Wahr, and F. O. Bryan, Climate-driven polar motion, J. Geophys. Res., 104, 12813-12829, 1999.

Chao, B. F., and Y.-H. Zhou, Meteorological excitation of interannual polar motion by the North Atlantic Oscillation, J. Geodyn., 27, 61-73, 1999.

Chao, B. F., W. O’Connor, D. Zheng, and A. Y. Au, Reply to Wunsch, J. Geophys. Int., 146, 266, 2001.

Chen, J. L., C. R. Wilson, R. J. Eanes, and B. D. Tapley, Geophysical contributions to satellite nodal residual variation, J. Geophys. Res., 104, 23237-23244, 1999a.

Chen, J. L., C. R. Wilson, R. J. Eanes, and R. S. Nerem, Geophysical interpretation of observed geocenter variations, J. Geophys. Res., 104, 2683-2690, 1999b.

Chen, J. L., C. R. Wilson, B. F. Chao, C. K. Shum, and B. D. Tapley, Hydrological and oceanic excitations to polar motion and length-of-day variation, Geophys. J. Int., 141, 149-156, 2000.

Cheng, M., and B. D. Tapley, Seasonal variations in low degree zonal harmonics of the Earth’s gravity field from satellite laser ranging observations, J. Geophys. Res., 104, 2667-2681, 1999.

Crétaux, J.-F., L. Soudarin, F. J. M. Davidson, M.-C. Gennero, M. Bergé-Nguyen, and A. Cazenave, Seasonal and interannual geocenter motion from SLR and DORIS measurements: Comparison with surface loading data, J. Geophys. Res., 107(B12), 2374, doi:10.1029/2002JB001820, 2002.

de Viron, O., V. Dehant, and H. Goosse, The “hidden torque”: The art, for a torque, to dominate everywhere and appear in no equation, in Vistas for Geodesy in the New Millennium, edited by J. Adám and K.-P. Schwarz, pp. 423-427, IAG Symposia vol. 125, Springer-Verlag, New York, 2002.

Foldvary, L., and Y. Fukuda, Evaluation of temporal variations on the gravity field caused by geophysical fluids and their possible detection by GRACE, in Gravity, Geoid, and Geodynamics 2000, edited by M. G. Sideris, pp. 143-148, IAG Symposia vol. 123, Springer-Verlag, New York, 2001a.

Foldvary, L., and Y. Fukuda, IB and NIB hypotheses and their possible discrimination by GRACE, Geophys. Res. Lett., 28, 663-666, 2001b.

Foldvary, L., and Y. Fukuda, Effects of atmospheric variations on the marine geoid determined by forthcoming gravity satellite, in Vistas for Geodesy in the New Millennium, edited by J. Adám and K.-P. Schwarz, pp. 187-192, IAG Symposia vol. 125, Springer-Verlag, New York, 2002.

Fujimoto, H., M. Mochizuki, K. Mitsuzawa, T. Tamaki, and T. Sato, Ocean bottom pressure variations in the southeastern Pacific following the 1997-98 El Niño event, Geophys. Res. Lett., 30(9), doi:10.1029/2002GL016677, 2003.

Fujita, M., B. F. Chao, B. V. Sanchez, and T. J. Johnson, Oceanic torques on solid Earth and their effects on Earth rotation, J. Geophys. Res., 107, 10.1029/2001JB000339, 2002.

Fukuda, Y., T. Sato, Y. Tamura and Y. Aoyama, The effect of sea-surface height variations on the long-period gravity changes, Bollettino di Geofisica Terica ed Applicata, 40(3-4), 511-517, 1999. 

Furuya, M., and Y. Hamano, Effect of the Pacific Ocean on the Earth's seasonal wobble inferred from National Center for Environmental Prediction ocean analysis data, J. Geophys. Res., 103, 10131-10140, 1998.

Gross, R. S., The excitation of the Chandler wobble, Geophys. Res. Lett., 27, 2329-2332, 2000.

Gross, R. S., Gravity, oceanic angular momentum, and the Earth’s rotation (invited), in Gravity, Geoid, and Geodynamics 2000, edited by M. G. Sideris, pp. 153-158, IAG Symposia vol. 123, Springer-Verlag, New York, 2001.

Gross, R. S., CHAMP, mass displacements, and the Earth’s rotation, in First CHAMP Mission Results for Gravity, Magnetic and Atmospheric Studies, edited by Ch. Reigber, H. Lühr, and P. Schwintzer, pp. 174-179, Springer-Verlag, New York, 2003.

Gross, R. S., I. Fukumori, and D. Menemenlis, Atmospheric and oceanic excitation of the Earth’s wobbles during 1980–2000, J. Geophys. Res., in press, 2003.

Gruber, Th., Ch. Reigber, and J. Wünsch, Estimation of ocean mass redistribution by means of altimetry and circulation models and its impact on the gravity field, in Towards an Integrated Global Geodetic Observing System (IGGOS), edited by R. Rummel, H. Drewes, W. Bosch, and H. Hornik, pp. 218-221, IAG Symposia vol. 120, Springer-Verlag, New York, 2000.

Höpfner, J., Atmospheric, oceanic, and hydrological contributions to seasonal variations in length of day, J. Geodesy, 75, 137-150, 2001.

Hughes, C. W., Torques exerted by a shallow fluid on a non-spherical, rotating planet, Tellus, 54A, 56-62, 2002.

Johnson, T. J., The role of the ocean in the planetary angular momentum budget, Ph.D. thesis, 134 pp., Univ. of Texas, Austin, 1998.

Johnson, T. J., The effects of the correction for mass non-conservation in global ocean circulation models on predicted oceanic angular momentum variability, EOS, Trans. Amer. Geophys. Union, 46, 80, 1999.

Johnson, T. J., C. R. Wilson, and B. F. Chao, Oceanic angular momentum variability estimated from the Parallel Ocean Climate Model, 1988-1998, J. Geophys. Res., 104, 25183-25195, 1999.

Johnson, T. J., P. Kammeyer, and J. Ray, The effects of geophysical fluids on motions of the Global Positioning System satellites, Geophys. Res. Lett., 28, 3329-3332, 2001a.

Johnson, T. J., C. R. Wilson, and B. F. Chao, Nontidal oceanic contributions to gravitational field changes: Predictions of the Parallel Ocean Climate Model, J. Geophys. Res., 106, 11315-11334, 2001b.

Johnson, T. J., B. J. Luzum, and J. Ray, Improved near-term UT1R predictions using forecasts of atmospheric angular momentum, J. Geodyn., in press, 2003.

Kakuta, C., T. Tsubokawa, and K. Iwadate, Coupling of long oceanic waves in the Pacific Ocean and the rotating elastic Earth during the 1986-1987 El Niño, J. Geophys. Res., 105, 3089-3094, 2000.

Leuliette, E. W., and J. M. Wahr, Climate excitation of polar motion, in Vistas for Geodesy in the New Millennium, edited by J. Adám and K.-P. Schwarz, pp. 428-433, IAG Symposia vol. 125, Springer-Verlag, New York, 2002a.

Leuliette, E. W., R. S. Nerem, and G. L. Russell, Detecting time variations in gravity associated with climate change, J. Geophys. Res., 107(B6), doi:10.1029/2001JB000404, 2002b.

Liao, D., X. Liao, and Y. Zhou, Oceanic and atmospheric excitation of the Chandler wobble, Geophys. J. Int., 152, 215-227, 2003.

Mangiarotti, S., A. Cazenave, L. Soudarin, and J. F. Crétaux, Annual vertical crustal motions predicted from surface mass redistribution and observed by space geodesy, J. Geophys. Res., 106, 4277-4291, 2001.

Marcus, S. L., Y. Chao, J. O. Dickey, and P. Gegout, Detection and modeling of nontidal oceanic effects on Earth's rotation rate, Science, 281, 1656-1659, 1998.

Nastula J., and R. M. Ponte, Further evidence for oceanic excitation of polar motion, Geophys. J. Int., 139, 123-130, 1999.

Nastula J., R. M. Ponte, and D. A. Salstein, Regional signals in atmospheric and oceanic excitation of polar motion, in Proceedings of IAU Colloquium 178, Polar Motion: Historical and Scientific Problems, Cagliari, Sardinia, Italy, ASP Conference Series, 463-472, 2000.

Nastula, J., R. M. Ponte, and D. A. Salstein, Regional high-frequency signals in atmospheric and oceanic excitation of polar motion, Adv. Space Res., 30, 369-374, 2002.

O’Connor, W., B. F. Chao, D. Zheng, and A. Y. Au, Wind stress forcing of the North Sea pole tide, J. Geophys. Int., 142, 620-630, 2000.

Petrov, S., A. Brzezinski, and J. Nastula, First estimation of the non-tidal oceanic effect on nutation, in Proc. Journées Systemes de Reference Spatio-Temporels 1998, edited by N. Capitaine, pp. 136-141, Obs. de Paris, Paris, 1999.

Ponte, R. M., A preliminary model study of the large-scale seasonal cycle in bottom pressure over the global ocean, J. Geophys. Res., 104, 1289-1300, 1999.

Ponte, R. M., and A. H. Ali, Rapid ocean signals in polar motion and length of day, Geophys. Res. Lett., 29(15), doi:10.1029/2002GL015312, 2002.

Ponte, R. M., and J. Dorandeu, Uncertainties in ECMWF surface pressure fields over the ocean in relation to sea level analysis and modeling, J. Atmos. Oceanic Technol., 20, 301-307, 2003. 

Ponte, R. M., and R. D. Ray, Atmospheric pressure corrections in geodesy and oceanography: A strategy for handling air tides, Geophys. Res. Lett., 29, doi:10.1029/2002GL016340, 2002.

Ponte, R. M., and D. Stammer, Role of ocean currents and bottom pressure variability on seasonal polar motion, J. Geophys. Res., 104, 23393-23409, 1999.

Ponte, R. M., and D. Stammer, Global and regional axial ocean angular momentum signals and length-of-day variations (1985–1996), J. Geophys. Res., 105, 17161-17171, 2000.

Ponte, R. M., D. Stammer, and J. Marshall, Oceanic signals in observed motions of the Earth's pole of rotation, Nature, 391, 476-479, 1998.

Ponte, R. M., D. Stammer, and C. Wunsch, Improving ocean angular momentum estimates using a model constrained by data, Geophys. Res. Lett., 28, 1775-1778, 2001.

Ponte, R. M., J. Rajamony, and J. M. Gregory, Ocean angular momentum signals in a climate model and implications for Earth rotation, Clim. Dyn., 19, 181-190, 2002.

Ponte, R. M., A. Mahadevan, J. Rajamony, and R. D. Rosen, Uncertainties in seasonal wind torques over the ocean, J. Climate, 16, 715-722, 2003.

Reigber, Ch., H. Jochmann, J. Wünsch, K. H. Neumayer, and P. Schwintzer, First insight into temporal gravity variability from CHAMP, in First CHAMP Mission Results for Gravity, Magnetic and Atmospheric Studies, edited by Ch. Reigber, H. Lühr, and P. Schwintzer, pp. 128-133, Springer-Verlag, New York, 2003.

Sato, T., Y. Fukuda, Y. Aoyama, H. McQueen, K. Shibuya, K. Asari, and M. Ooe, On the observed annual gravity variation and the effect of sea surface height variations, Phys. Earth Planet. Inter., 123, 45-63, 2001.

Stammer, D., R. Tokmakian, A. Semtner, and C. Wunsch, How well does a 1/4° global circulation model simulate large-scale oceanic observations?, J. Geophys. Res., 101, 25779–25811, 1996.

Thomas, M., J. Sündermann, and E. Maier-Reimer, Consideration of ocean tides in an OGCM and impacts on subseasonal to decadal polar motion excitation, Geophys. Res. Lett., 28, 2457-2460, 2001.

Wahr, J., M. Molenaar, and F. Bryan, Time variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE, J. Geophys. Res., 103, 30205-30229, 1998.

Wünsch, J., Oceanic influence on the annual polar motion, J. Geodyn., 30, 389-399, 2000.

Wünsch, J., Oceanic and soil moisture contributions to seasonal polar motion, J. Geodyn., 33, 269-280, 2002a.

Wünsch, J., Erratum to “Oceanic and soil moisture contributions to seasonal polar motion”, J. Geodyn., 34, 709-710, 2002b.

Wünsch, J., M. Thomas, and T. Gruber, Simulation of oceanic bottom pressure for gravity space missions, Geophys. J. Int., 147, 428-434, 2002.



7. International Earth Rotation Service
Jan Vondrak, Chairman of the Directing Board
Bernd Richter, Director of the Central Bureau

The new organization of the IERS

After ten years of activity it became necessary to reorganize the IERS for various reasons: the re-organization of the space techniques SLR, Lunar Laser Ranging, VLBI, and GPS into International Services (DORIS being now organized as a pilot program) and the development of a new component: the Global Geophysical Fluids Center (GGFC) dealing with motion of the various fluid layers and their relationship with reference frames and Earth dynamics. The re-organization of the Service began in November 1999, when the Call for Participation was issued. The Letters of Intent were reviewed by the IERS Directing Board (DB) at its meeting in December 1999 (San Francisco). It was decided to set up a Proposal Review Committee (PRC) with the task to evaluate all proposals and prepare the corresponding recommendations for the next IERS DB meetings. The PRC was composed of knowledgeable scientists, under the chair of I.I. Mueller, at the beginning of 2000. Its first recommendations were discussed at the IERS DB meeting in Washington (June 2000), and some of them accepted. Nevertheless, there were still several multiple proposals for the same components. The primary scientists of these were further contacted by the PRC and, in some cases, new joint proposals were asked for. The IERS DB was able to take final decisions at its meeting in Frankfurt a.M. (September 2000). Several minor changes of the Terms of Reference were adopted by the 'old' Directing Board at its last meeting in San Francisco (December 2000), mainly reflecting a slightly changed structure of the ITRS Product Centers. The new structure of the IERS began operation on January 1, 2001.
The main changes in IERS structure are: The previous Sections and Sub-Bureaus of the Central Bureau are now autonomous components within the IERS and are called Product Centers; The Central Bureau moved from Paris to Frankfurt am Main in Germany and has now primarily administrative functions; New elements of the structure are Combination Research Centers, ITRS Combination Centers and the Analysis Coordinator; External International Services (like IGS, ILRS and IVS) serve as Technique Centers of the IERS.
According to the IERS Terms of Reference, the primary objectives of the IERS are to serve astronomical, geodetic and geophysical communities by providing: International Celestial Reference System and Frame (ICRS, ICRF); International Terrestrial Reference System and Frame (ITRS, ITRF); Earth Orientation Parameters (EOP) that define the transformation between the ICRS and ITRS; Relevant geophysical data (i.e., information on the distribution and motion of the atmosphere, terrestrial and oceanic water, mantle, core...); Conventions (i.e., standards, constants, models, algorithms, software...).
To cover this broad field of interest and to realize the products, the new IERS Terms of Reference define the following components of the new IERS:
Technique Centers (TC) that are generally autonomous independent services, cooperating with the IERS. There is typically only one TC per technique, and it provides its operational products to the IERS. At the moment, these are the following: International VLBI Service (IVS); International GPS Service (IGS); International Laser Ranging Service (ILRS); International DORIS Service (IDS) that has however not yet been formed; the technique serves as a Pilot Experiment of the CSTG.
Product Centers (PC) that are responsible for the products of the IERS. They are: Earth Orientation PC, responsible for monitoring long-term orientation parameters, publications for time dissemination and announcements of leap seconds. It is located at Observatoire de Paris. Rapid Service/Prediction PC, responsible for providing Earth orientation parameters on a rapid basis, primarily for real-time users. It is placed at U.S. Naval Observatory, Washington D.C. Conventions PC is responsible for the maintenance of the IERS conventional models, constants and standards. Joint proposal of U.S. Naval Observatory (Washington D.C.) and Bureau International des Poids et Mesures (Sèvres) was accepted. International Celestial Reference System PC, responsible for the maintenance of ICRS and its realization, ICRF. Joint proposal of Observatoire de Paris and U.S. Naval Observatory was accepted. International Terrestrial Reference System PC, responsible for the maintenance of ITRS and its realization, ITRF. It is located at IGN, Marne-la-Vallée. Global Geophysical Fluids PC, responsible for providing relevant geophysical data sets and related results. This center now consists of eight Special Bureaus (for Atmosphere, Core, Gravity/Geocenter, Hydrology, Loading, Mantle, Oceans, and Tides), and is hosted at NASA's Goddard Space Flight Center.
Combination Centers: ITRS Combination Centers, responsible for providing ITRF products by combining ITRF inputs from the Technique Centers and other sources. Three institutions established ITRS Combination Centers: DGFI, Munich, Germany; Geomatics Canada, Ottawa, Canada; IGN, Marne-la-Vallée, France. Combination Research Centers, responsible for the development of combinations from data (or products) coming from different techniques. They are expected to provide their solutions to Analysis Coordinator. There are now eleven of them AICAS & CTU, Prague, Czech R.; FGS & DGFI, Munich, Germany; FGS & FESG, Munich, Germany; FGS & GIUB, Bonn, Germany; GFZ, Potsdam, Germany; FFI, Kjeller, Norway; GRGS, Toulouse, France; IGN, Marne-la-Vallée, France; JPL, Pasadena, USA; IAA, St. Petersburg, Russia; ASI, Matera, Italy.
Analysis Coordinator (Markus Rothacher, TU Munich, Germany) is responsible for long-term and internal consistency of the IERS reference frames and other products, for ensuring the appropriate combination of the TC products into a single set of official IERS products and for archiving them.
Central Bureau (placed at BKG, Frankfurt a. M., Germany, under the direction of Bernd Richter) is the administrative center of the IERS. It is responsible for the general management (according to the directives given by the Directing Board), for coordinating the activities, IERS publications, archiving the products and it also serves as the communication center with the users. The CB presently designs a data center to store and archive all products which are necessary for the IERS products and also those which are necessary to re-compute the products.
Directing Board that exercises general control over the activities of the IERS. The chairperson, elected by the Board from its members (Jan Vondrák, Astronomical Institute, Academy of Sciences of the Czech Republic, Prague), is the official representative of the IERS to external organizations. The DB consists of two representatives of each of the Technique Centers, one for each of the Product Centers, one for all Combination Research Centers together, a representative of the Central Bureau, Analysis Coordinator, and the representatives nominated by the IAU, IAG/IUGG and FAGS.

IERS Activities in 2000-2003

The IERS DB in its new composition met at least twice a year: Meeting No. 30 in Washington, June 3-4, 2000; No. 31 in Frankfurt a.M., September 14-15, 2000; No. 32 in San Francisco, December 18, 2000; No. 33 in Nice, March 26, 2001; No. 34 in Brussels, September 26, 2001; No. 35 in Paris, April 20, 2002; No. 36 in Munich, November 22, 2002; and No. 37 in Paris, April 2, 2003 to decide on important matters of the Service like structural changes, overall strategy, creating working groups, launching projects, changing Terms of Reference, etc.

The IERS organized two Workshops and a Retreat: The first was the IERS Workshop on the Implementation of the IAU 2000 Resolutions (Paris, April 18-19, 2002). It included detailed presentations and explanations of the contents of the Resolutions, the background reasons for their adoption, associated concepts and implementations as well as answers to specific questions. About 80 participants from 20 countries took part in the meeting. The Proceedings were printed as IERS Technical Note No. 29 and are available also online through IERS's website. The results of this workshop are, to some extent, reflected in IERS Conventions 2000, and also in new IERS products (celestial pole offsets referred to new precession-nutation model IAU2000A), published since the beginning 2003. The second workshop was the IERS Workshop on Combination Research and Global Geophysical Fluids (Munich, November 18-21, 2002). Its main goal was to improve all IERS products in accuracy, consistency, stability, timeliness, user-friendly access, and documentation, and to make first steps towards a rigorous combination of the various products, contributing thus significantly to the realization of an "International Global Geodetic Observation System" (IGGOS). The workshop was divided into two strongly related parts: different aspects of the comparison and combination of the results of all major space geodetic techniques; IERS Global Geophysical Fluid Center (GGFC), the present status of its products and its role in future. The Proceedings of this Workshop will be published as IERS Technical Note No. 30.

The IERS Retreat (Paris, March 31 - April 1, 2003) was summoned to review the present and propose future IERS products, ensure their better mutual consistency, to specify the IERS requirements and to develop a vision for the future. Discussion on possible organizational consequences led to the concrete proposals for the DB meeting that immediately followed the Retreat.
Among the most important decisions made by the DB in 2000-2003 belong: Creation of the new ITRS Combination Center in Canada; Creation of the new GGFC Special Bureau for Loading; Creation of the new Combination Research Center in Italy; Decision to contribute to the prepared IAG Pilot Project IGGOS; Launching the IERS Combination Pilot Project; Establishment of Working Groups: on Combination, the ITRF datum, and Site co-location; Changing the name to International Earth rotation and Reference systems Service, without changing the acronym (IERS).



8. Permanent Service for Mean Sea Level (PSMSL)
Phil Woodworth, Director

Introduction 
The PSMSL is operated at the Proudman Oceanographic Laboratory (POL), Bidston Observatory under the auspices of the International Council for Science (ICSU), and is a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS). The PSMSL reports to the International Association for the Physical Sciences of the Ocean Commission on Mean Sea Level and Tides (IAPSO/CMSLT) and has an Advisory Board consisting of scientists expert in each area of sea level research. Annual reports on the work of the PSMSL are circulated each year to the International Association of Geodesy (IAG), the Intergovernmental Oceanographic Commission (IOC), IAPSO, FAGS, and other relevant bodies and are available publicly via the web at: http://www.pol.ac.uk/psmsl/ This same web page also serves as a source of PSMSL data and ancillary information.

PSMSL Staff 
Mr. Graham Alcock took early retirement in 2000. Graham was closely involved in PSMSL and GLOSS matters for over 20 years, being the main organiser of over 10 GLOSS training courses, having represented PSMSL and GLOSS at many international meetings, and having authored several important GLOSS reports. Dr. Philip Axe also left PSMSL and POL in 2000 to take up an appointment in Sweden and is now based at SMHI.. Of particular note during 2001 was the MBE (Member of the British Empire) awarded by the Queen to Mrs. Elaine Spencer in the 2001 New Year’s Honours List. Elaine was PSMSL Technical Secretary between 1974 and 1999. The development of the PSMSL data set formed part of the citation for the award. Two new scientists joined the PSMSL in 2002 following increased funding received from the UK Natural Environment Research Council. The first was Dr. Svetlana Jevrejeva from Tallinn in Estonia, who has published a number of papers on climate variability including studies of sea ice changes in the Baltic related to the North Atlantic Oscillation. The second was Dr. Simon Holgate from Liverpool University who has a background in sea-level related geology, geography and palaeo-carbon flux studies. Svetlana and Simon will lead the PSMSL data collection in future, with some continued assistance from Mrs. Rose Player. 

PSMSL Data Receipts for 1999-2003 
On average, approximately 1500 station-years of data were entered into the PSMSL database during each year of the period. This compares well to rates obtained in previous years. Most data originated from Europe, North America and Japan, but all regions are represented in the receipts at some level. Important data gaps in South America, Africa and parts of Asia are receiving special attention as part of the JCOMM GLOSS programme (see below). Figure 1 indicates the locations from which data were received during 1999-2003. Comparison to the corresponding figure produced in the PSMSL report for the IUGG in 1999 shows slightly fewer stations this time in Africa and South America and considerably more in the Arctic. The main method for distribution of PSMSL data is now unquestionably the internet, almost all other methods now having been abandoned. However, regular CDs and now DVDs are produced as backups and for people in some countries without good web access. A DVD was produced in 2002 with the PSMSL data set as part of the final conference of the World Ocean Circulation Experiment (WOCE), containing all tide gauge data collected during the programme. 

GLOSS Activities 
The Global Sea Level Observing System (GLOSS) is a project of the Joint Commission for Oceanography and Marine Meteorology (JCOMM) of the Intergovernmental Oceanographic Commission (IOC) and World Meteorological Organisation (WMO). One of the main aims of GLOSS is to improve the quality and quantity of data supplied to the PSMSL. GLOSS has been one of the first components of the Global Ocean Observing System (GOOS). GLOSS network status as perceived by the PSMSL is reviewed each year and can be found at http://www.pol.ac.uk/psmsl/programmes/gloss.info.html while a review of progress within the programme has been prepared by the PSMSL as a ‘GLOSS Adequacy Report’ submission to the 2003 IOC Assembly. Meetings of the GLOSS Group of Experts, which is the management committee for the programme, have been held every two years alongside scientific and technical workshops. GLOSS training courses have been held in many countries since the mid-1980s. Since 1999, courses have been held in Brazil (1999), Saudi Arabia (2000), Guatemala (2001), India (2003), Chile (2003) and Malaysia (2003). (The Guatemala and India courses were not official IOC-funded GLOSS courses but had significant GLOSS participation). The GLOSS training programme also includes web based materials, training manuals, newsletters and tidal software. As an example of training materials, the PSMSL with some support from GLOSS funded Dr. Glenn Milne of the University of Durham to produce maps of sea level change through the Holocene period, showing the changes in coastlines which resulted. A joint project between the IOC International Oceanographic Data and Information Exchange (IODE) Committee, GLOSS and PSMSL to conduct a ‘data archaeology’ survey of historical sea level records, was begun by Dr. Lesley Rickards. This project has the aim of extending existing time series and gaining access to observations which are not in digital form. 

European Projects 
A European Union (EU) funded sea level study called SELF-2 for the Mediterranean was completed during the period with PSMSL and POL participation, with concentration at POL on mean sea level changes, storm surge modelling, absolute gravity and tidal loading. The EU EOSS project aimed to enhance sea level (tide gauges) and land level (GPS) monitoring, and associated data exchange in Europe, primarily by sets of bilateral (i.e. no new cost) agreements. That project ended in September 2001 with an international conference in Dubrovnik, Croatia, and was followed by Calls for Participation in a new European Sea Level Service (ESEAS) which it is hoped will continue and extend the work of EOSS, and put the provision of sea and land level information from Europe on a sounder basis. In 2000, Dr. Woodworth attended the first Coordination Meeting of the MedGLOSS programme at Haifa, Israel organised by Dr. Dov Rosen. MedGLOSS is a joint programme of the International Commission for the Scientific Exploration of the Mediterranean Sea (CIESM) and IOC and aims to install and coordinate a network of gauges for the Mediterranean and Black Seas. 

Altimetry and Gravity Field Activities 
Participation has continued in European and US altimeter working groups during the period. Dr. Woodworth is a Principal Investigator for the TOPEX/POSEIDON and JASON-1 missions and of particular interest to the PSMSL is the symbiosis between altimetry and tide gauge measurements with gauges being used extensively by the project to calibrate the altimeter data set. During 2001, Dr. Xiaojun Dong from the Shanghai Astronomical Observatory joined the sea level group at POL through a Fellowship from the Royal Society, with the object of researching the best methods for ongoing altimeter calibration using tide gauge data. This resulted in one paper being accepted for publication in Marine Geodesy with other work in progress. Drs. Woodworth and Hughes of POL have during the period been members of the Mission Advisory Group (MAG) of the European Space Agency (ESA) Gravity Field and Steady State Ocean Circulation Experiment (GOCE) mission which is planned for launch in 2006. This is a major development for ocean circulation and sea level studies in the next decade. Drs. Hughes and Woodworth are also involved in the use of data from the US-German Gravity Recovery And Climate Experiment (GRACE). 

Geodetic Fixing of Tide Gauge Benchmarks 
In 1997, an important meeting on tide gauge benchmark fixing was held at the Jet Propulsion Laboratory, prior to the fifth meeting of the GLOSS Experts (GE5). This meeting was organised jointly by the IGS Central Bureau, the PSMSL and IOC/GLOSS and resulted in an excellent workshop report on the use of GPS at gauge sites for measuring long term changes in vertical land movements and for altimeter calibration. In 1999 and 2001, follow-up meetings were held in Toulouse, France and Honululu, USA alongside GE6 and GE7 respectively. In September 2002, a study week was organised on vertical crustal motion and sea level change and on the use of GPS at tide gauges in Toulouse. The week included the development of the TIGA project at GFZ, Germany which aims to better understand the uncertainties in the use of GPS in this role further, and was held under the auspices of the IGS/PSMSL/IAPSO/IAG/GLOSS CGPS@TG working group which had been formed at the 1997 JPL meeting. As part of CGPS@TG work, regular surveys have been conducted on behalf of the PSMSL, EUREF and other organisations on the availability of permanent GPS stations near to tide gauges by Dr. Guy Woppelmann of the University of La Rochelle. 

Publications 
The PSMSL has a responsibility to not only collect and redistribute sea level information, but also to analyse data and publish scientific results. The main papers published each year are listed in PSMSL Annual reports. However, three important ones may be mentioned here. The Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) was published during 2001 with Chapter 11 on sea level changes led by Dr. J. Church (Australia) and Dr. J. Gregory (UK) and with Dr. Woodworth as a Lead Author. In 2002 a major paper was published by Dr. Woodworth and others on the use of tide gauges during WOCE. Finally, a review paper of the work of the PSMSL was published in the Journal of Coastal Research in 2003. 

GLOUP . 
The PSMSL is responsible to the IAPSO Commission on Mean Sea Level and Tides for the maintenance of the data base of pelagic (bottom pressure recorder) information. This data base, called GLOUP (Global Undersea Pressures), was maintained during the period by Dr. Chris Hughes and can be inspected at: http://www.pol.ac.uk/psmslh/gloup/gloup.html 

Other PSMSL Activities
Mr. Philip Knight of POL has written a newsgroup type software called the ‘PSMSL Forum’ which allows discussion via emails of matters of importance to the PSMSL, such as developments in tide gauge technology or sea level research. The software is presently being tested by volunteers and the utility of such a forum will be assessed during 2003. The opportunity has been taken whenever possible to publicise the work of the PSMSL in newspapers and on radio and TV. Presentations were given in the period in all three media in several countries and details can be found in the PSMSL Annual Reports. Plans have advanced for POL’s relocation from Bidston Observatory to a new building on the campus of Liverpool University in September 2003. This will include the relocation of the PSMSL. Our new postal address and phone and fax numbers will be advertised on the PSMSL web pages as soon as possible but our email and web addresses will be unchanged. We expect that any disruption to the work of the PSMSL will be temporary. It can be seen that 1999-2003 has been a further active period with regard to important workshops and conferences, and a busy one with regard to data acquisition and analysis. Particular thanks as usual go to PSMSL staff, and also to the staff of the Proudman Oceanographic Laboratory who provide the extended Service. 


9. BIPM Time Section
Felicitas Arias, Head

International time scales
Reference time scales International Atomic Time (TAI) and Universal Coordinated Time (UTC) have been computed regularly and have been published in the monthly BIPM Circular T. Definitive results for 1999, 2000, 2001 and 2002 have been available, in the form of computer-readable files from the BIPM website (http://www.bipm.org), and on printed volumes of the respective Annual Reports of the BIPM Time Section. Work has been done to automate the calculation of the time scales TAI and UTC published monthly in the BIPM Circular T. Several modifications have been introduced in Circular T, starting in May 2003: results are given to 0.1 ns; the list of time links used in the calculation of the current circular is provided; and a new format has been adopted for the layout of the circular.

Algorithms for time scales
Research concerning time scale algorithms includes studies to improve the long-term stability of the free atomic time scale EAL and the accuracy of TAI. The weighting procedure of clocks participating in TAI has been revised and modified. Until 31 December 2000, the maximum relative weight of clocks participating into TAI was fixed to 0.700%. With the improvement of the commercial atomic clocks, this method became inefficient to discriminate between the best clocks. Since January 2001 the maximum relative clock weight has been established each month, depending on the number of participating clocks. This modification improves the stability of the international time scales.The medium-term stability of EAL, expressed in terms of the Allan deviation, is estimated to be 0.6 ´ 10-15 for averaging times of 20 to 40 days over the period. 
Primary frequency standards developed and operated by the National Institute of Standards and Technology (NIST, USA), the Communications Research Laboratory (CRL, Japan), the Paris Observatory (OP, France) and the Physikalish-Technische Bundesanstalt (PTB, Germany) reported their measures to the BIPM. The global treatment of these individual measurements led to a relative departure of the duration of the TAI scale unit from the SI second on the geoid ranging, in 2002, from +6 ´ 10-15 to +11´ 10-15, with an uncertainty smaller than 3 ´ 10-15. The frequency offset between TAI and EAL is changed when necessary; this operation is referred to as the “steering of TAI”. Following the recommendations of the Consultative Committee on Time and Frequency, changes were implemented to render the data used in TAI, as well as the results, more accessible to the users and to make the procedures of calculation even more transparent and traceable. Since April 2000 two modifications had been implemented: a new model to characterise the instability of the free atomic scale EAL, and a more complete representation of the uncertainty of the deviation of the TAI scale interval relative to that of the Terrestrial Time TT.

Time links
Two techniques of time transfer are used at present to compare clocks in TAI: GPS common-views based on C/A measurements and two-way satellite time and frequency transfer (TWSTFT). In the last decade the time links computed at the BIPM used the classical GPS common-view technique based on C/A-code measurements obtained from single-channel receivers. The commercial availability of newly developed receivers has stimulated interest in extending the classical common-view technique for use of multi-channel dual-code dual-system (GPS and GLONASS) observations, with the aim of improving the accuracy of time transfer. Since July 1999 GPS multi-channel links and TWSFTF links have been progressively introduced in TAI. Ionospheric maps and precise operational satellite ephemerides provided by the International GPS Service (IGS) are routinely used to correct all links in regular TAI calculations since May 2000. In addition, the BIPM Time section carries on research on new techniques of time transfer, such as the utilisation of geodetic type receivers. These activities had been developed in the period 1999-2002 in the frame of the IGS/BIPM pilot project to study accurate time and frequency comparison using GPS phase and code measurements, and have been incorporated to the IGS as a current activity since 2003. A pilot experiment (TAIP3) was proposed in April 2002 to laboratories participating to TAI. The goal was to study time links computed with GPS P3 data obtained from geodetic-type dual-frequency receivers. Comparisons of such P3 time links with other techniques presently used for TAI have been conducted in the aim of evaluating the long term stability of P3 time links and, as a matter of fact, to assess the long-term stability of the other techniques. It has been concluded that the reliability and long-term stability of P3 links are adequate for their use in TAI computation.

Space-time references
The BIPM/IAU Joint Committee on general relativity for space-time reference systems and metrology (JCR), concluded his work in January 2001. Its activities have been undertaken by the IAU working group on Relativity, Celestial Mechanics, Astrometry and Metrology (RCMAM). Studies have been conducted at the BIPM in collaboration with other members of the JCR/RCMAM, they concern the extension of the relativistic framework to allow a correct treatment for time transformations and the realisation of barycentric coordinate time at the full post Newtonian level and the realisation of geocentric coordinate times. Since January 2001 the BIPM, jointly with the USNO, is the Conventions Product Centre of the International Earth Rotation Service (IERS).



10. International Center for Earth Tides
B. Ducarme, Director

The staff of ICET, which is completely supported by the Royal Observatory of Belgium, our host Institution, is composed as follows:Prof. B.Ducarme, Director(part time);Mrs. L.Vandercoilden, technician(full time);Mr. M.Hendrickx, technician(part time). The Royal Observatory of Belgium has hosted ICET since 1958 and continues to provides numerous administrative and scientific facilities especially for the publication of the “ Bulletin d’Information des Marées Terrestres” (BIM), for the tidal data processing and since 1997 for the maintenance of the ICET/GGP data base.

Terms of reference
The terms of reference of the International Centre for Earth Tides(ICET) can be summarised as follows: as World Data Centre C, to collect all available measurements on Earth tides; to evaluate these data by convenient methods of analysis in order to reduce the very large amount of measurements to a limited number of parameters which should contain all the desired and needed geophysical information; to compare the data from different instruments and different stations distributed all over the world, evaluate their precision and accuracy from the point of view of internal errors as well as external errors; to help solving the basic problem of calibration by organising reference stations or realising calibration devices; to fill gaps in information and data; to build a data bank allowing immediate and easy comparison of earth tides parameters with different Earth models and other geodetic and geophysical parameters ; to ensure a broad diffusion of the results and information to all interested laboratories and individual scientists. These goals are achieved essentially by the diffusion of information and software, the data processing, the training of young scientists and the welcome of visiting scientists.

Main Commitments
It appears first that most geodetic measurements are affected by earth tides, as at the centimetric level the tidal displacement of the station is no more negligible. It will thus remain an important task for ICET to provide algorithms for tidal computation or analysis. For example the geophysicists, such as seismologists or volcanologists, who are measuring crustal deformations for natural hazards monitoring, are now conscious of the necessity of dealing properly with the tidal signals. In a similar way absolute gravity measurements require accurate tidal corrections that should take into account the local tidal parameters. These parameters have to be computed including oceanic tidal loading effects or even require in situ tidal gravity observations.
On the other hand the earth tidal scientific community is limited. The last International Symposium on Earth Tides, held in Mizusawa, Japan from August 28 to September 1st, 2002 , brought together only a bit more than one hundred and twenty participants. The groups are always very small and often marginally involved in tidal research. The papers dealing specifically with tidal studies are not fitting so well to international journals. It is thus very important to keep a specialised diffusion and information medium. It is the vocation of the “Bulletin d’Information des Marées Terrestres”(BIM). ICET is generally publishing two eighty pages issues per year.
Besides this basic activity, which is the scientific challenge for the beginning of this century? 
The mathematical modelisation of the astronomical tidal forces as well as the elastic response of the Earth made decisive progress. It is now possible to model the astronomical tidal forces to within 5 nanogal in the time domain. The different mathematical techniques for the evaluation of the tidal response of the Earth do agree now to better than 0.1%. The most recent models include inelasticity in the mantle.
The last problems to be solved are linked to the fluid elements of our planet: liquid core resonance, oceanic loading, meteorological effects, underground water.
Among the ground based observations only gravity tides are able to give informations valid at the regional level. The other components(tilt, strain, volume change) are heavily depending of the local parameters of the crust, including cavity or topography effects. These observations should be mostly used to monitor tectonic deformations after removing the tidal phenomena.
Tidal gravity observations are able to provide constrains on the liquid core resonance by means of very precise observations in selected sites. The same is valid also for the selection of the most realistic model for the elastic or inelastic response of the Earth. For that purpose it is essential to improve the calibration methods in order to achieve a 0.1% accuracy in amplitude and a 0.01o accuracy in the phase determination. It is also necessary to use up to date oceanic tides models for tidal loading corrections. The determination of the amplitude factor of the polar motion effect on gravity will constrain the Earth viscosity at low frequency.
To achieve these goals it will be necessary to tackle three main questions: oceanic tidal loading, atmospheric pressure effects, underground water. It is only possible through a coordinated effort and a multidisciplinary approach including Astronomy, Geodynamics, physical Oceanography, Hydrology and Climatology..

Ongoing projects
These objectives are now directly addressed by the “Global Geodynamic Project”(GGP). A network of 20 stations equipped with cryogenic gravimeters is in operation since July 1997, using a similar hardware and the same procedures for data acquisition. A first 6 years term finishes in July 2003 and a second term 2003-2009 will start immediately after.
Besides tidal research, an important objective of GGP is to study the residues after elimination of the tidal contribution in order to detect inertial accelerations such as free oscillations of the Earth core and mantle with periods larger than 50 minutes, which are difficult to observe by means of conventional seismometers. In fact the cryogenic gravimeters are extra-large band instruments covering phenomena with period ranging from one second to more than one year. GGP is a unique opportunity to obtain high quality well calibrated tidal observations, and ICET has been interested to support this project since its beginning. ICET is responsible for the "Global Geodynamics Project-Information System and Data Centre" (GGP-ISDC, http://etggp.oma.be/). The data owners can upload themselves the original minute sampled data. The data are carefully preprocessed at ICET using a standard procedure, to correct for tares and spikes. The data are then decimated to one hour and analysed. The analysis results are directly communicated to the data owners. This follow up is required to detect quickly the anomalies that could affect the data. Each year CD-ROM's are edited with the raw and corrected minute data as well as the log files and the auxiliary data, when available. The members of the GGP consortium can download data older than one year. After two years the data are fully opened to the public.
The archiving of the data is rather complex as the data are only released according to a strict time table. The data are sent to ICET only one year after their production. During one additional year the data are only available to the GGP members and can be freely accessed only after two years. The software provided for the management of GGP-ISDC by the GeoForschungZentrum Potsdam is continuously updated. With the collaboration of guest scientists ICET pushed forward researches using the GGP data sets and concerning the liquid core resonance, the determination of the pole tide and the detection of the inner core oscillations known as Slichter's mode. We have now more than 20 high quality data sets with a minimum length of three year and we can provide on request not only tidal parameters, oceanic loading corrections according to different models but also tidal residues to study non tidal effects such as core modes. These series, if they are well constrained by absolute measurements, will be also useful in the interpretation of satellite gravity data. To improve the tidal loading corrections ICET gathered the most recent oceanic tides models.

Ongoing Activities
The “Bulletin d’Information des Marées Terrestres”(BIM) is printed in 300 copies.Some 275 copies are sent to libraries and individual scientists all over the world. It is devoted to scientific papers concerning tidal research. From May 2000 until October 2002, six issues n° 132 to 137 have been published with a total number of 720 pages. In 2002 we had the opportunity to publish the proceedings of the "Third Workshop of the Global Geodynamics Project (GGP) on superconducting Gravimetry" and of the "Meeting of the ETC-Working Group 7 on Analysis of Environmental Data", held in Jena, Germany, from March 11 to 15,2002. For the first time all the published papers were immediately available on the ICET WEB site.
ICET made an agreement with Marion Wenzel, wife of late Prof.H.G.Wenzel, who inherited the property rights on the ETERNA tidal analysis and prediction software. ICET is now authorised to distribute freely this software among the scientific community for non commercial purposes. This initiative met a great success as some forty CD-ROMS with ETERNA software are requested from ICET each year since May 2000.
The ICET WEB site (http://www.astro.oma.be/ICET/) has been updated and developed. Besides general information including historical aspect and last ICET reports, it proposes to the visitors an access to: the general bibliography on Earth Tides from 1870-1997 either by alphabetical order of the first author or following the decimal classification introduced by Prof. P.Melchior; the table of content of all the previous BIM, n° 1-137, and starting from BIM 133 an electronic version of the papers; tidal analysis and preprocessing software available from different WEB sites or on request from ICET. Most of the information requests (one per week minimum) concerned software. Most of them followed the consultation of the WEB site. This site is one of the most frequently consulted among the pages of the Royal Observatory of Belgium (ROB), which is the host agency. 
According to the internal GGP rules ICET is preparing annually CD-ROM's, with the raw and processed minute data. We already edited CD-ROM's for the 4 first years, 1997/07 to 2001/06, of the project. 

Visitors
ICET welcomed more than 20 visitors. Besides visitors coming only for a short stay we must consider also guest scientists and trainees. The guest scientists bring their own know how or data to work at ICET during several weeks or even months. Some of them worked on the ICET and GGP data banks, as Dr. A.Kopaev( Sternberg Astronomical Institute, Moscow), Prof. H.P. Sun and his assistants (Institute of Geodesy and Geophysics, CAS, Wuhan, China), Prof. A.P. Venedikov (Institute of Geophysics Sofia). Others brought their own data sets to perform tidal analyses using the ICET software and computing facilities, as Dr. E.Boyarski and Prof. L.Latynina (Institute of Physics of the Earth, RAS, Moscow), Prof. L.Brimich (Geophysical Institute, SAS, Bratislava Slovakia), Prof Silvia S.Schwab ("Universidade Federal de Parana", Curitiba, Brazil), Dr. V.Timofeev( Institute of Geophysics, UIGGM, Novosibirk, Russia). Mrs P.Beddows (School of Geographical Sciences, Bristol University, UK) came to receive intensive training on earth tide data processing and analysis.

Summer School
In the framework of the International Gravity Field Service (IGFS) recently created inside IAG, a summer course on "Terrestrial Gravity Data Acquisition Techniques" has been organised jointly by ICET and the "Bureau Gravimétrique International" (BGI), with the support of IAG and FAGS. Some 45 students from 27 countries took part to this school that took place on the campus of the Catholic University of Louvain in Louvain-la-Neuve, Belgium from September 4 to 11. A CD-ROM with all the teaching material is under preparation. The aim of this summer school was the training in gravimetric techniques of people involved in geodesy, geodynamics, geophysics or geology. At the end of the school, they were able to operate relative gravimeters and handle gravity data in order to realise gravity networks, densification surveys or microgravimetric studies. Special attention was paid to the tidal gravity corrections. There were three different types of activities: lectures, field practice and case studies. The case studies were talks given by specialists who are using the gravimetric techniques in Geodesy and Geophysics or for civil engineering applications. 

Planned developments
Following the creation inside IAG of the International Gravity Field Service (IGFS), ICET will deepens its cooperation with the other confederated bodies. We believe that the members of IGFS should develop a common WEB site to stress their complementarity. Given the connections already developed with the International Gravimetric Bureau (BGI) we shall try a common experience at a smaller scale. For that purpose, according to the new FAGS funding policy, in line with the new ICSU strategy, we presented a bid for funding together with the BGI for the development of a common WEB site. With an improved WEB site it will be possible also to convert partly the "Bulletin d'Information des Marées Terrestres" into an electronic journal. It will speed up the publication of the papers, which are generally dedicated to ongoing researches, and reduce drastically the costs. Another possibility offered inside IGFS is to appoint "Fellows", which are individual scientists wishing to contribute to the Service activities. It will be possible to develop a network of contributors who can provide their expertise to ICET in answering to very specialised questions, developing new software and so on.